Pretty much this is the order of my thought process when flying around an area with enemy contacts.
1) Are they higher or lower than me? And are they climbing (Gaining Potential Energy at a loss for Kinetic) or diving (Gaining Kinetic Energy at a loss for Potential)
2) How fast are they going? And in what direction? [Sometimes, because of their heading and alt it looks like they are going "slow" when really they are rocketing around. Just take a second to observe.]
3) What plane are they in, and how does its engine, and energy retention compare to mine, as well as maneuverability throughout varying speeds? Essentially know what each plane is capable of at varying e-states
4) At what alt, and how fast am I going?
Some tips for learning how to establish all this.
1) Whenever you see a 262 (Or 163) in a fur ball (Friendly or Enemy) just watch them for a minute and they will provide you a good look at what a plane looks like when they are going 400-600mph!
2) When examining a co-alt con at a far distance (Usually out of icon range) look at his relationship to the horizon as well as your noses relationship to the horizon to establish this ( One thing I have noticed is that often times an enemy co-alt with you will *appear* to be slightly lower than you at a distance when really they are co-alt or slightly higher, usually about ~1-1.5k higher than you. )
3) Try not to rely too much on the icon distances, they can be very very deceptive.
But as the Glzsqd noted, you just kinda have to observe and learn through trial and error.
Judging E states is the first part of situation awareness, it is observation of the body language of the plane, similar to a cop or race car driver observing the body language of a car. As you watch fights unfold, you will begin to notice certain movements as certain maneuvers or directional changes. A lot of the times while in a fur-ball, I can tell you exactly which plane is looking and whom based on positioning and energy states (of their plane and others) way before that plane starts its dive or makes it obvious who they are attacking.
Don't just focus on your enemies e-states, also look at your friendlies E states as well. Look at who is engaged with who, who is looking at who, who can potentially attack who, try to see the battle unfolding before it does!