Detecting when the user taps on an object?

Hello. I would like to detect whenever the player taps on an object in my iOS game. I know the obvious solution would be to just have a button be directly above it. Although this works, I am going several (fast moving) instances of this object on screen at a time and I have already had some issues with the Unity GUI slowing down my game. Is there any alternative solution? Thanks

You need to raycast every frame that the user touches the screen. There are some scripts on the Wiki that convert that into the OnMouseDown message that is sent by the mouse in standalone.