You should really replace anything that's had a heavy impact, even if you can't see a crack.
Modern helmets work by the liner de-forming in response to pressure - it compresses to take the impact and doesn't spring back. If that is between the liner and shell you'll never see it unless you cut the helmet in half. But it leaves a weak spot, which won't protect you properly should you be unlucky enough to hit your head in the same place again.
Hey - let's be careful out there. :)