Yeah I usually worry the same, especially when I am stoned because it is that much more precious to me, hahaha. Anyway, now I usually can tell just by looking. Also, when you try to hit it, and a big fucking flaming ball of ash comes flying through, it is done. Good idea to blow across the top, assuming you down blow everything out. But basically give it a shot, worse comes to worse, you spit out the grossness, or nothign happens. Hit it till it is dead. Oh and ALWAYS let the next person know if you think there is either not much left, or if it is kicked, give fair warning like "Yo this might be kicked, but you can give it a shot if you want." and if they say no, ask anyone else.