The primary byproducts of fermentation are ethanol and CO2. This means bubbles in the airlock are a good indicator that fermentation is going on. However, like most things, presence might prove, but absence does not necessarily disprove. By this I mean, if you see bubbles and krausen you can be sure your yeast are active and working away, however, a lack of bubbles and krausen is not proof that your yeast are not working.
The best way to tell if fermentation is complete is to check gravity.