BHSI.org says the following:
Did you drop it hard enough to crack the foam? Replace.
For starters, most people are aware that you must replace a helmet
after any crash where your head hit. The foam part of a helmet is made
for one-time use, and after crushing once it is no longer as
protective as it was, even if it still looks intact. Bear in mind that
if the helmet did its job most people would tell you that they did not
even hit their head, or did not hit their head that hard. And the thin
shells on most helmets now tend to hide any dents in the foam. But if
you can see marks on the shell or measure any foam crush at all,
replace the helmet. (Helmets made of EPP foam do recover, but there
are few EPP helmets on the market. Yours is EPS or EPU unless
You can also crack the helmet foam or damage it by dropping the helmet
on a hard surface. The cracks may be small and hard to see, so you
need to look carefully. Cracks in the foam always require replacement
of the helmet.
You may be reluctant to replace a helmet that looks almost as good as
new, but if you did hit, you don't want to take chances on where you
will hit next time. If the foam is cracked under the thin shell, it
will be more likely to fly apart in your next crash. Many
manufacturers will replace crashed helmets for a nominal fee, and most
will also inspect crashed helmets to see if they need replacement.
Call them if you are in doubt.