You can tell REALLY easily when the bowl is ashed. The smoke should taste like smoke(not like pot smoke) and be really really harsh. It like burns the back of your throat the second you breathe it in.
If you're talking about the rez though, which is all the black sticky stuff left after you go through a couple bowls, that will get you pretty fucked up if you smoke it. The way i use to tell if my bowl is rezed up enough to smoke is if when I'm done with a bowl and i try to blow it out it sticks to the bowl and i have to pick it out.