Re: how do you know?
Simply you can take your foil off.
You could take a poker or a needle or something and poke though a hole you already made and test for crunchyness.
Or gain more experience and taste and feel that the tobacco is done without having to do the former 2 suggestions.
burnt tobacco is usually black and dry and crunchy.
"blu mist rulz dawg an that stuff ain't like it an if youz don't agree you bez a shithead yo" - Hajo Flettner