The picking and selection tutorials supplied in other answers aren't applicable here; amongst other glSelectBuffer and glRenderMode aren't part of OpenGL ES.
There are, as stated elsewhere, essentially two approaches. You can figure out how the touch maps into your scene and test your objects mathematically. If your square is flat on, as though you were drawing it in 2d, then you can achieve that quite easily.
Alternatively, you can do something essentially like picking but adapted for the functionality available under OpenGL ES. For example, clear your scene to black and draw your square in solid blue. Use glReadPixels to get the colour value at the touch location. If it's blue then the touch is on the square. This is probably a more expensive way to test because it means an additional draw of the scene, but it means that you don't have to implement any additional maths or other code beyond what GL supplies.
The only slight thing to keep in mind is that OpenGL uses the same way of addressing pixels everywhere — (0, 0) is the bottom left, positive y goes up and positive x goes left. So whatever axes you have your touch coordinates relative to, you need to transform them to be relative to that before doing the glReadPixels.
