The short answer is NO! Please don't rely on the airlock as the sole means of monitoring fermentation.

Quite often it is incorrectly assumed that fermentation has not commenced because the airlock is not bubbling, when in fact the brew is fermenting and CO2 gas is escaping via a poor sealing lid, or the airlock has a split in it, or the grommet may need replacing. This answer assumes that the correct temperature range is being maintained for the yeast you are using.

Visual signs of establishing whether the brew did, in fact, start fermenting are
* Signs of scum on the inside of the fermenter above the brew surface
* Signs of froth forming on top of the brew
* Hydrometer readings taken over a few days have dropped from the first reading.

If you have answered YES to any of the above do nothing as your brew is fermenting.
If you have answered NO to all of the above, sprinkle in a new yeast and close everything up.

Helpful hint :- if your fermenter has a screw on lid take off the lid, using your finger smear a small amount of honey around the top of the fermenter, reseal the lid and half fill the airlock with clean water.