The main way is through 'situational awareness'; if you are flying the aircraft, you should have an idea of what is it doing based on what it has done, and what you have commanded it to do. For example, if it is flying from right to left as you look and you command a right turn, then when the plane appears to have no horizontal movement it is going away from you.
However, it is common and easy to briefly lose this awareness. In this case, I would take what I can see and attempt a manoeuver that I know the outcome of:
- If the aircraft looks like it is either coming towards me or going away, I will roll slightly - if I command 'right wing down' and the right wing goes down, I am looking at the aircraft from behind. If the left wing (as I see it) dips, it is 'backwards' - i.e. coming towards me. (Of course, you could also use yaw and see which way the silhouette goes.)
- If the aircraft is possibly flying inverted, I will use either elevator to lift/dip the nose or rudder to yaw the plane:
- If it is flying left to right and I try to push the nose down but it goes up, the plane is inverted. (I would not expect to see a yaw movement very well in this case.)
- If it is flying away from me and I try to yaw left and it goes left, it is the right way up. (Up/down movement would probably be visible in this orientation, but I prefer maintaining altitude.)
I've also used simulators, and find that it is also slightly easier to work this out in real life as things like depth perception help a lot.
Finally, it is worth mentioning that if you are flying in a country that requires you to keep the aircraft in "Visual Line of Sight" (or VLOS), if your aircraft is so far away that you cannot easily identify it's orientation visually, it is probable that you are no longer flying in VLOS conditions (as you cannot see what your aircraft is doing, even though you can see the aircraft.)