It is important to understand, how the helmet works, in the first place. If there is nothing (heavy) in the helmet, when it falls down, from a bike, it will not damage the foam, which actually protects your head, slowing it down, during impact.
I believe, if an empty helmet falls, and there is no damage (besides scrapes) to the outer shell, the foam is not affected. If you have something in the helmet, when it drops, you should take the padding out and inspect the foam, for any compression.