I just look down and then sit a beat if I'm off. I've been riding english for over 15 yrs and I still can't pick up the correct lead 100% of the time. I do pick it up correctly the majority of the time, but I always visually check it by a quick peek at the leg.
Oh I had a hard time with this when I started posting, too. My best advice is to get your horse moving out a bit more and then try sitting the first few beats, then start to post. I found if I waited a beat or two I would pick it up correctly. If your horse has a slow western jog this will be more difficult. Even after riding for years I still have to correct my diagonal on occasion.
What I do is I feel for the outside front leg's movement. Try to picture in your mind when the horses leg goes forward, and just rise with it. I'll sit for a couple of beats while I figure out when that leg is moving forward, and once I find the rhythm I post. It might help to have someone lunge you at the trot and close your eyes to really pay attention to the rhythm.
I'm trying my best to explain how I get it, but I'm pretty sure that isn't all that much help haha. All I know is that is what I do and I get it 100% of the time without having to look.
Good luck :] & I'll try to think of better ways to explain it.