Alrighty, i know its a big no-no to look down when riding, but I learned my leads by asking for the canter and looking at the wither. Use your peripheral vision to observe the shoulders. If the Inside shoulder (shoulder not against the rail) seems to be moving forward further than the other shoulder, then you are on the correct lead. If the outside shoulder seems to be bulging further forward than the inside, then you are on the incorrect lead.
When observing a horse cantering, the leg that outstretches the furthest is the lead that the horse is performing. So if the front right leg is the 3rd leg to hit the ground, and stretches out further than the other front leg, the horse is on the Right Lead.
I hope that makes sense o_o as far as riding goes, as Poptart said, you'll really feel when the horse is on the incorrect lead, especially as you go into a corner. The horse will not want to bend in the corner and will usually break to the trot, or do a flying lead change to get back on the correct lead. Most of the time they just break to the trot though, lol... its lazier that way.
I hope that helps and it made sense, LOL im better with talking, not typing :p recognizing leads is easy once you figure out what you're lookin at!