Getting the "feel" for diagonals can take some time, so lots of practice! Try closing your eyes/not looking for a few strides, try to feel and guess whether you're right and then check.
Try to watch other riders and see ignore you can tell what diagonal they are on. If you identify that it should be easier for you!
It's fine, happy to help!!! 😜
You should be able to somewhat feel if your not on the right diagonal, but if your still not sure just check to see if your rising when your pony/horses outside shoulder is forward.