I've tried simply dotting a mob with one dot at a time, and sometimes it seemed to me that they crit occasionally, but other times they don't crit; the dot does the same damage over and over, and I wind up thinking that I imagined that it DID crit, formerly.
I need to know, because I levelled as an affliction lock, then went demo/destructo at 70, just to try it out... and because I was sick of mobs in instances dying well before the dots had done much, and me always winding up on the bottom of the DPS damage charts. So I started collecting crit gear for 70 epics... but I'm kind of really missing several of the affliction spells, and thinking of respeccing back to affliction, especially if it turns out the fights are longer in heroic instances and raids.
The thing is, it's probably better not to respec to affliction if dots don't crit (in the same way that arcane bolts crit), given the gear I have now.