How to know what UI element is being touched?

You can get EventSystem.current and change the position to touch position. Then all the raycastes you get with the RaycastAll function will give you the gameObjects that you are currently touching.

 public bool TouchOnTrashButton()//if touch phase ends on this button 
 {                               //than the function will return true .
     if (Input.GetTouch(0).phase == TouchPhase.Ended)
     {
         PointerEventData eventDataCurrentPosition = new PointerEventData(EventSystem.current);
         eventDataCurrentPosition.position = new Vector2(Input.GetTouch(0).position.x, Input.GetTouch(0).position.y);

         List<RaycastResult> results = new List<RaycastResult>();
         EventSystem.current.RaycastAll(eventDataCurrentPosition, results);
         if (results.Count > 0)
         {
             if (results[0].gameObject.name.Equals("TrashCanGFX"))
             {
                 return true;
             }
         }
     }
     return false;
 }