As Sofia said. It just takes time to develop the feel, same as feeling your leads at the canter. Some good exercises to start feeling your diagonal are sitting the trot down the centre line, closing your eyes, picking a direction and starting posting on what you think will be the right diagonal. You can then open your eyes and check. You can build this into your rides as a fun warm up exercise.
When I first started riding I had to look, but now I can tell just by feeling. It just takes time and attention, I think.