It's difficult to say exactly what a particular horse's canter command is, but there are usually a few common cues. Because so many people train horses so many different ways, sometimes cues can be confused. Learning the canter cue for your particular horse might take a little experimenting but there are a few common cues you should try first.
The most common cue for canter is slippng the outside leg back a few inches and pressing into the horse's barrel. This cues the horse to push off with that ouside hind leg into the canter. For some well trained horses, this cue is only meaningful at the correct point in their stride when the outside hind is about to land, but for most horses applying this aid at any time is a command for them to strike off into canter at the next stride.
Other common commands are a slight thrust with the pelvis towards the inside shoulder (given at the same time as the command with your leg) and slack given on the inside rein.
Verbal commands for canter may include a smoochy, kissy, or clucky sound. Or simply the word "canter".
If you're having trouble locating your horses command for canter, you just may have to train a new one!