so how do you know its a soldier and how it is a marine?
Dead enemy body count?
From uniform nowadays. Marines have digital desert/woodland/snow and army has that grey one.
Yep, that they're finally phasing out cuz they finally realized that grey is not camo.
Also, the marines wear the distinctive traditional dress blues uniform:
While army wears the crappy business suit uniform, made slightly better recently by the decision to change back to blue and to change back to shoulder boards for the officers: