I'm making a 2D mobile platformer and I just figured out how to make the mobile buttons work. But I can't get the jump limit to work. Does anyone know how to create maybe a ground detection with this script?

 Rigidbody2D playerRigidbody = null;
 
 public GUITexture buttonTexture = null;
 
 void Start()
 {
     playerRigidbody = playerObject.GetComponent<Rigidbody2D>();
 }
 
 void Update()
 {
     TouchInput (buttonTexture);
 }
 void OnFirstTouchBegan()
 {
     switch (buttonType) 
     {
     case type.JumpButton:
         playerRigidbody.AddForce (Vector2.up * jumpHeight, ForceMode2D.Impulse);
         break;
     }
 }
 
 void OnSecondTouchBegan()
 {
     switch (buttonType) 
     {
     case type.JumpButton:
         playerRigidbody.AddForce (Vector2.up * jumpHeight, ForceMode2D.Impulse);
         break;
     }
 }
 
 void OnFirstTouch()
 {
     switch (buttonType)
     {
     case type.LeftButton:
         playerObject.transform.Translate(-Vector2.right * moveSpeed * Time.deltaTime);
         break;
     case type.RightButton:
         playerObject.transform.Translate(Vector2.right * moveSpeed * Time.deltaTime);
         break;
     }
 }
 
 void OnSecondTouch()
 {
     switch (buttonType)
     {
     case type.LeftButton:
         playerObject.transform.Translate(-Vector2.right * moveSpeed * Time.deltaTime);
         break;
     case type.RightButton:
         playerObject.transform.Translate(Vector2.right * moveSpeed * Time.deltaTime);
         break;
     }
 }

}