It's random. Yeah.
It's not an exact science, but in the right light, you can tell by the biggest gun. Seen a big loop strap, that's the ARC trooper rifle. See a big barrel, that's the EMP launcher (sometimes you make out it's distinct stock, as well), and the thick strap or thick solid ended barrel is sandtrooper rifle. If you see the clone trooper rifle that only tells you that it's not the sandtrooper pack, as the other two have the clone rifle.
Also, the one commando gun is gun metal color, not black, it's the only weapon that's not black out of all of them.