Most of us learned long ago, do not wear a uniform off base on your own time. That has always been true and remains so today.
In the case of Marines generally the haircut gives you away.
Marines don't have to be in uniform to be recognized as Marines. These are the distinguishing characteristics: very short haircuts, relative youth, obvious good physical condition, military bearing, always at least two together, and, almost always, politeness. There are Marines in the Hawk & Dove every night. It is a place where "my little pony" Capitol Hill staffers go to meet real men.