It is somewhat hard to explain, because part of it is the relative rates and angles to what your own airplane is doing. You are not looking at the displays except through the HUD. You let your ass tell you what your plane is doing while you watch the other guy. After awhile, you can just look at a guy and know. There are visual clues like that big-ass vapor that the Hornet and Viper throw from their wing roots under G because of their wing extensions, and other craft throw vapor too, just not so consistantly. My old bird had the biggest giveaway because my wings would be moving up as I got slow.(we would take them off the computer to trick Air Force guys) You could see if a guy is plowing, lots of deflection, not a lot of movement, and a few other things. Look at the Raptor in the picture, he's clean, no vapor, you can see no deflection in the control surfaces. The Raptor has big-ass surfaces. When the Raptor brakes, it deflects its ailerons, and turns its rudders out. If it was accelerating, I'd be looking for some burner out back. So its easy to see he's just cruising in the photo.
Does that help?
Yes, you explained it well. It actually makes sense. Like you said, I can see how a person would eventually just sense what the other one is doing by all those tell tale clues.