I can never tell by taste when it's done. I guess it tastes worse and worse as it goes, but there's no specific change when it gets cached. Maybe it makes up for being a "super-taster" and having to taste that godawful shit in the paper strips in chemistry class.
The problem with knowing when a bowl is cashed by sight and lighter application alone is that some bowls are so rezzed up that you can get a hit of rez off of ash which in my opinion is gross as fuck. I like to tap the bowl with a lighter, and then blow on it gently if I'm not sure.
When you first light depending on how packed it is, it will turn brown or black, but its not done! just keep lighting and moving around the lighter so that it hits the sides and shit. When it looks like ash from a ciggeritte, then its done.