There's too many "tells," waaaay too many to list. I just mostly key in on the hair and makeup, posture, and if they're in ACU"s or BDU's, I focus on badge placement. Especially the name and Branch badges. They always get that backwards.  Those are the only one's I'll list, because quite frankly, imo, the civilians don't have a right to know. ONLY prior Military have the knowledge to successfully find these fuckers out.