before you add one to the y of your sprite (the tetris tile) use the point function to check where you're heading. now, say you're nowhere near the bottom/other tiles. that means the pointing will return 0, for the colour 0. (the colour the screen is) so you add y and loop again. however, if you're about to hit a tile/the bottom, the pointing will return a number other than 0. then you stop handling that sprite and start handling another. if done right, this can be very accurate.
sorry for not giving you any code or explaining too well. i'm kinda rushing.