>>2223388
All I can suggest is (1) look up the aircraft type and learn as much as you can about its publicly described mission. Also who bought it and when it was put into service, etc. (2) Then notice that aircraft everytime it pops up and become familiar with its typical doings. (3) If it does anything out of the usual, like flying extremely fast from point A to point B, high- or low-altitude flight, accompanied flight, etc. then it becomes something interesting to speculate about what it might be doing.
Usually there's no way to tell. RedArrow has far more experience than me and anytime he's around head over to Planefagging 101 and start asking questions. RA has a wealth of information.
He promoted me recently, but I don't feel like I'm living up to the stripes b/c trying to do so many things at once and doing none of them well.