For a MARS tactical situation they might abbreviate their callsigns;
AA6RYU becomes 6RY, you get the idea.
For NATO aligned militaries, one callsign convention is LFL, such as N0S, P3A, W4J, etc. I'd have to listen to them to see if they're MARS or mil.
If they give an id for the net you can tell what branch it is if MARS thus;
SERVICE/NET ID
FLFL = USN/MC
LFL = Army/AF
QRG DESIGNATION CALLSIGN PREFIX
LFFF = USA NNN = USN/MC
LLL = USN/MC AA_ = USA
AFA = USAF