The taste will tell you and sometimes the density of the smoke exhaled (not always) can also be a sign the bowl is done or almost done. If you didn't break it up or it got packed down hard there can be a small amount of unburned green underneath the ash.

Hopefully when you have a little more experience you can tell after you hit the bowl that you just hit something that didn't make you any higher too. That's always a good sign it's done unless you're so stoned you can't tell but that's why you'll learn that with some time under your belt.