Unity появление текста при наведении на кнопку

Вы можете реализовать в своем классе специальные интерфейсы, а конкретно вам нужен IPointerEnterHandler. Вот сокращенный код из моего проекта, где обрабатываются события нажатия кнопки мыши (не клик), наведение курсора и убирание курсора с элемента:

public class ButtonBase :
    MonoBehaviour,
    IPointerDownHandler,
    IPointerEnterHandler,
    IPointerExitHandler
{
    ...
    public void OnPointerDown  (PointerEventData eventData) { ... }
    public void OnPointerEnter (PointerEventData eventData) { ... }
    public void OnPointerExit  (PointerEventData eventData) { ... }
}

Если же вы не хотите писать свою кнопку, а вас устраивает та, что уже есть, посмотрите компонент EventTrigger. Он позволяет ставить обработчики на разные события прямо в редакторе.

Сильно не пинайте. Когда то давно делал контроллер для всплывающих подсказок. В нескольких проектах его использовал.

using System;
using UnityEngine;
using System.Collections;
using System.Reflection;
using UnityEngine.EventSystems;
using UnityEngine.UI;
public class ToolTipController : MonoBehaviour
{
    private enum TipState
    {
        None,
        Old,
        New
    }
    public GameObject LastObject = null;
    public float Delay = 0.0f;
    public Text Label = null;
    public GameObject Background = null;
    public float ResearchTime = 2.0f;
    public bool AutoHide = true;
    public ToolTip DefaulTip = null;
    public GameObject TipObject
    {
        get; private set; }
    private ToolTip toolTip = null;
    private string toolTipText = "";
    private Camera[] cameras   ;
    private float timer = 0.0f;
    private Vector3 lastMousePosition = Vector3.zero;
    private GameObject LastObjectUI = null;
    private static ToolTipController instance;
    private float waitResearch = 0.0f;
    public static ToolTipController Instance
    {
        get { return instance; }
    }
    void Awake()
    {
        if (instance != null)
            Debug.LogError("The scene has got more than one tooltip controller.");
        instance = this;
    }
    // Use this for initialization
    public void Start ()
    {
        ResearchCameras();
        //Hide();
    }
    public void ShowTip(string tip)
    {
        toolTip = null;
        toolTipText = tip;
        Show();
    }
    public void ShowTip(ToolTip tip)
    {
        toolTip = tip;
        toolTipText = null;
        Show();
    }
    // Update is called once per frame
    public void Update ()
    {
        if (lastMousePosition != Input.mousePosition && Delay > 0)
        {
            timer = 0.0f;
            lastMousePosition = Input.mousePosition;
            LastObject = null;
            Hide();
            return;
        }
        else
        {
            if (timer < Delay)
            {
                timer += Time.deltaTime;
                return;
            }
        }
        TipState newState = TipState.None;
        if (LastObjectUI != null)
        {
           newState = (TipState)Mathf.Max((int)newState, (int)GetTargetUI());
        }
        else
        {
            foreach (Camera cam in cameras)
            {
                newState = (TipState)Mathf.Max((int)newState, (int)GetTargetCamera(cam));
            }
        }
        switch (newState)
        {
            case TipState.None:
                if (DefaulTip != null)
                {
                    toolTip = DefaulTip;
                    toolTipText = null;
                    TipObject = toolTip.gameObject;
                    Show();
                }
                else {
                    Hide();
                }
                LastObject = null;  
                    break;
            case TipState.New:
                Show();
                break;
        }
    }
    public void ResearchCameras()
    {
        cameras = FindObjectsOfType(typeof (Camera)) as Camera[];
    }
    TipState GetTargetUI()
    {
        if (!LastObjectUI.gameObject.activeInHierarchy)
        {
            LastObjectUI = null;
            return TipState.None;
        }
        return GetTarget(LastObjectUI);
    }
    TipState GetTargetCamera(Camera cam)
    {
        // create the ray on the mouse position from camera
        Ray ray = cam.ScreenPointToRay(Input.mousePosition);
        RaycastHit hit;
        if (!Physics.Raycast(ray, out hit))
            return TipState.None;
        GameObject currentObject = hit.collider.gameObject;
        return GetTarget(currentObject);
    }
    TipState GetTarget(GameObject fromObject)
    {
        if (fromObject == null) return TipState.None;
        if (fromObject == LastObject) return TipState.Old;
        GameObject currentObject = fromObject;
        do
        {
            toolTip = currentObject.GetComponent<ToolTip>();
            if (toolTip != null)
            {
                LastObject = fromObject;
                TipObject = fromObject;
                Show();
                return TipState.New;
            }
            else
            {
                Component[] components = currentObject.GetComponents<Component>();
                foreach (Component component in components)
                {
                    Type type = component.GetType();
                    FieldInfo fieldInfo = type.GetField("Tip");
                    PropertyInfo propertyInfo;
                    toolTipText = fieldInfo != null ? fieldInfo.GetValue(component) as string: "";
                    if (string.IsNullOrEmpty(toolTipText))
                    {
                        fieldInfo = type.GetField("tip");
                        toolTipText = fieldInfo != null ? fieldInfo.GetValue(component) as string: toolTipText;
                    }
                    if (string.IsNullOrEmpty(toolTipText))
                    {
                        propertyInfo = type.GetProperty("Tip");
                        toolTipText = propertyInfo != null? propertyInfo.GetValue(component, null) as string: toolTipText;
                    }
                    if (string.IsNullOrEmpty(toolTipText))
                    {
                        propertyInfo = type.GetProperty("tip");
                        toolTipText = propertyInfo != null ? propertyInfo.GetValue(component, null) as string: toolTipText;
                    }
                    if (!string.IsNullOrEmpty(toolTipText))
                    {
                        LastObject = fromObject;
                        TipObject = fromObject;
                        Show();
                        return TipState.New;
                    }
                }
            }
            if (currentObject.transform.parent != null) 
                currentObject = currentObject.transform.parent.gameObject;
            else 
                currentObject = null;
        } while (currentObject != null);
        return TipState.None;
    }
    void Show()
    {
        if (Background != null)
            Background.SetActive(true);
        if (Label != null)
        {
            Label.text = toolTip != null ? toolTip.Text: toolTipText;
            Label.gameObject.SetActive(true);
        }
    }
    void Hide()
    {
        if (!AutoHide) return;
        if (Background != null)
            Background.SetActive(false);
        if (Label != null)
            Label.gameObject.SetActive(false);
    }
    public void OnEnter(BaseEventData data)
    {
        PointerEventData pData = data as PointerEventData;
        if (pData != null && pData.pointerCurrentRaycast.gameObject)
        {
            LastObjectUI = pData.pointerCurrentRaycast.gameObject;
        }
    }
    public void OnExit(BaseEventData data)
    {
        PointerEventData pData = data as PointerEventData;
        if (pData != null && pData.pointerEnter)
        {
            LastObjectUI = null;
            Hide();
        }
    }
}

На сколько помню надо было либо в одном из компонентов задать в скрипте текст подсказки в поле/свойстве Tip/tip. Либо навешать на объект скрипт.

using System.Collections.Generic;
using System.Runtime.Remoting.Messaging;
using UnityEngine;
using System.Collections;
using UnityEngine.Events;
using UnityEngine.EventSystems;
using UnityEngine.UI;
public class ToolTip : MonoBehaviour
{
    [Multiline(5)]
    public string Text = "";
    public uint MaxWidth = 0;
    public Text Output = null;
    public float TimeDisplay = 0.0f;
    private bool isUI = false;
    // Use this for initialization
    void Start () {
        isUI = this.transform is RectTransform;
        if (isUI)
        {
            EventTrigger trigger = gameObject.AddComponent<EventTrigger>();
            trigger.triggers = new List<EventTrigger.Entry>();
            EventTrigger.Entry entry = new EventTrigger.Entry();
            entry.eventID = EventTriggerType.PointerEnter;
            entry.callback = new EventTrigger.TriggerEvent();
            UnityAction<BaseEventData> callback = ToolTipController.Instance.OnEnter;
            entry.callback.AddListener(callback);
            trigger.triggers.Add(entry);
            entry = new EventTrigger.Entry();
            entry.eventID = EventTriggerType.PointerExit;
            entry.callback = new EventTrigger.TriggerEvent();
            callback = ToolTipController.Instance.OnExit;
            entry.callback.AddListener(callback);
            trigger.triggers.Add(entry);
        }
    }
    // Update is called once per frame
    void Update () {
    }

}

Обрабатывались как объекты в сцене с коллайдерами, так и UI компоненты