One thing I found (very much like what Caoimhe said) was looking at their leg, getting on the right diagonal, then on the wrong diagonal and finding out what it feels like for you and you’re horse so you can then eventually identify it straightaway 😊😊
Practice, practice and more practice. Have someone on the ground point it out to you until you can feel it. Always check the outside foreleg as you move into trot. Eventually it becomes automatic because you can feel within a stride if the diagonal is wrong.