The protection you get from your helmet depends on it's structural integrity.
To assess the damage after a fall, you would have to take into consideration, among other things, how cold was it, how hard did you hit, what you hit... did you fall on soft snow or did you run into a tree, and what was it's condition to begin with.
The problem is that helmets can sustain damage from an accident and have the damage invisible to the naked eye. Even a minor defect can potentially cause catastrophic structural failure the same way a chip in a car windshield can cause the entire windshield to shatter just by hitting a hard bump. This can also happen with cold plastic, although I'm sure they have designed around that as much as possible.
Back in the olden times, after an accident you could take your motorcycle helmet to an ER and they would x-ray it for you so you could tell if it was damaged. I would say liability claims have probably killed those days, and with the price of a ski helmet it wouldn't be cost effective even if they would do it for you.
All that said, I believe the standard most helmet manufacturers use is "after a significant impact". Personally, I would say if I hit my head hard enough to rattle it good, I'd replace my helmet no matter what it looked like. Other than that, I probably do more damage to my helmets by piling them in the trunk and throwing gear on top of them anyway.
Keep in mind, I'm not a lawyer or an equipment manufacturer or helmet safety expert, and this is just my personal opinion.