Often, you can have the mfg check it. Hard to say, but you certainly could have compromised it's ability to absorb impact. That's one of the reasons that mfgs recommend replacing your helmet on a schedule.

For example, my son had a skiing crash in an Uvex full coverage racing helmet. He was in a coma for three weeks. Obviously, the helmet took a shot and was no longer usable but it looked just fine under casual, lay, inspection - no scratches, cracks etc.... You'd have to measure the foam density throughout the helmet to know how much was crushed down. Often the damage is obvious, but just as often it isn't.

J.