Sit the trot, and start posting with the outside shoulder. If you pick up the wrong one, sit and try again. If you keep practicing this, eventually you will be able to feel the correct diagonal without even looking at the shoulder!
Start with a sitting trot and let it bounce you up onto your correct diagonal! That’s how I learned. Of course outside shoulder going forward is when you go up, and do you can check it, but the sitting trot starts you off to feel the diagonal
Once you’ve done it for long enough you can usually feel when you’re posting wrong, it kinda feels like you’re imbalanced. But if you cant feelt it just quickly glance down at the outside leg or in a mirror if you have one :)