You'll know if your not on the right lead, it'll feel super weird and uneven, the horse won't want to circle either . If you look at the inside shoulder the leg should be stretching farther then the others, that's when your on your correct lead. To correct just bring them down to trot and ask for the cante right away, they should catch on after a while .
You turn his head in and inside leg
You can look at the inside front shoulder, and also the canter will be bumpier if you're on the wrong lead.
A major way of telling if your horse is on the correct lead is how the canter feels- whether it is smooth or jerky/uncomfortable in turns. Using strong canter transitions and transition aids could help with picking up the correct lead.