Can you clarify: Are you having trouble getting your horse to take the correct lead? Or are you having trouble FEELING what lead your horse is on?
I'm going to assume that you are having trouble getting your horse to take the correct lead.
First of all, is your horse 100% broke? What I mean by that, is can you:
--walk, trot, and canter your horse on a loose rein and relaxed and collected
--stop softly from any speed (using primarily your seat and legs)
--back up freely
--control the head, neck, shoulders, ribcage, and hips (using your seat and legs mostly)
In a nutshell, you need to be able to have 100% control of your horse's body.
I like to start by teaching my horses a simple lead in a circle. Let's say we are going to pick up the LEFT lead where the horse's right hind leg strikes the ground first, followed by the left hind and right front together, and lastly by the horse's right left foot. It is called the left lead because the left front foot is "leading".
Make a decently-sized circle to the left. Get your horse trotting nice and collected and listening to you. I will pick up my left rein and tip their nose slightly to the left. I make an upward twisting motion with my hand to keep the shoulder elevated. I keep my left leg OFF the horse. I apply my right leg back a bit, and I bump them with my calf. This pushes their hip inward and prepares them to get their right hind leg underneath them. When I feel they are ready and in position, then I will slightly lean my upper body forward and I will give them a kissing sound to actually go into the lope (I have taught them ahead of time on the lunge line that the kissing sound means to lope).
I bolded the word FEEL because that's very important. When you first teach this, there are times when the horse's body is not in the right place and they will not be successful in getting the lead. And there are times when they are ready and they will pick it up correctly. This is something that you are going to have to learn by trial-and-error and with experience.
I won't kiss to my horse until I feel they will be successful with the lead departure.
If the horse does get it wrong, no big deal. Bring them back down to a trot, make a few small circles to the left, and then ask again. When they get it right, along them to keep loping along and praise them.
Of course, work this both ways, right and left. Once you master the trot-to-lope transition, then you can try it from the walk-to-lope, and eventually from a standstill to a lope.
Always keep your cues consistent and they same. Eventually, you will not need to use the reins at all to tip your horse's nose. You'll simply be able to use your legs and your body language. And of course, you'll eventually be able to do this on a straight-away and not in a circle.
As far as feeling if you are on the correct lead, that takes time and experience and FEEL. If you need to look down to check, that's okay. Just make sure you don't lean your weight forward when you do. I taught myself to feel leads by really exaggerating how much my hips swing with the horse. If you are on a left lead, you will feel your hips swing from your back/right to your front/left. And of course, opposite for the other lead. Some horses are easier to feel than others.
Now I know you said that you have a show in a couple weeks. Don't try to rush training. If you are ready, great. But if not, go for the fun and experience anyway. Lead changes need to be taught correctly from the beginning so that you horse learns a good solid cue.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.