I can tell when it feels like everything is going to the outside of the circle. It can depend on your horse as well if they are smooth you might notice it's rough and hard to keep your seat when on the wrong lead. Have a friend watch you they can tell you wrong lead but don't shut down right away and fix it ride it and feel it find out what wrong feels like then get the correct lead and feel it once you learn to feel you won't need to look.
A feel for your lead Is something that you develop over time. I can feel when they lift their hind end and from what side and if they're disunited, but that's a sense that has taken me years.
Try trotting/jogging on the centreline and picking up your lead without looking. Then say out loud what lead you think you're on and then check if you're right, turn the way you picked up, and take a second to feel what that lead feels like. Then trot and repeat.
Also play around with trying to pick up specific leads on the centre line, and see if you can feel the way they lift their shoulders for each one.