What you actually wanna look for and observe is tanker aircraft. Air superiority aircraft F-16, F-15. F-22, F-35 ect.. usually don't show up unless low level near class C airspace or something where they need to turn on transponders. They always need fuel though so tankers are a tell tale sign of more interesting stuff. Then you need aircraft scanners to try to find coms if your close enough.

What you actually wanna look and observe is tanker aircraft. Air superiority aircraft F-16, F-15. F-22, F-35 ect.. usually don't show up unless low level near class C airspace or something where they need to turn on transponders. They always need fuel though so tankers are a tell tale sign of more interesting stuff. Then you need aircraft scanners to try to find coms if your close enough.