Color is the best indicator. [edit; and taste. Burnt popcorn is right.
]
As graywulf pointed out, a nice golden brown, perhaps a shade or two darker than his avatars hair color, is a good indication of most of the 'goodies' being spent.
Evil1 said:
is that correct or can vapour-like "smoke" still be seen when the good stuff is finished ?
There are still plenty of actives that you can release after a bowl is sapped to the aforementioned state, consequentially though you are going to get some of those 'nasties' released as well (if I recall correctly; benzene, toluene, and PAH's at the highest temperatures. Don't quote me.).
It's in a way, a double edged sword. You'll still make clouds, and produce a heavier high, but at a certain point the 'health' aspect isn't what it is at lower temperatures. Water filtration is a good idea if your going to be vaping at higher temperatures.
Hope this helps.