If he always canters on the right lead, and struggles with the left, then I'd say it would be fairly conclusive as to which lead he is on if you're travelling to the right and he is cantering solidly, he's going to be favouring his right lead. On the left I'd assume he's be quite unbalanced if he struggles with it? If you're going to the right and he's on the left lead, a horse that struggles to pick up canter to the left is not going to have a balanced counter canter, making it quite conclusive as to which lead he is on???
Try cantering a 3 loop serpentine. Pick it up to the right, his stronger side, and canter a full circle, then start your serpentine. Notice that your inside hip will be working harder than your outside (note what KateS said). As you change rein to go to the left, your horse will be in counter canter and you will notice that you feel slightly awkward as your outside hip should still be more forward when instinctively you're inside hip wants to lead.
If you really can't tell, try putting your horse onto a 15m circle. If he falls apart he's probably on the wrong lead!