Give your helmet a look over every so often. Look at the lining and pads, make sure they’re in good shape. Lift the lining and look at the EPS—the surface has to be smooth and even. If it’s becoming porous or if there are noticeable dents, you might want to consider changing lids. Same goes if the outer shell is badly dented and/or cracked.
When you wear the helmet, if it wiggles freely when you move and doesn’t follow the motions of your head, that’s also a sign it’s time to go helmet shopping.