How to fix jumping exploit Unity

Hello I have a player controller and some simple obstacles in my scene like cubes and such. I have found an exploit where if you get close to a wall and spam the jump button, you will go flying in the air. I know what I need to do to fix it, but I don't know how. I need to add a sort of check to see if I'm colliding with the top face of the cube, not the sides. How can I do that?