Service members come from every walk of life. Just because someone doesn't walk around looking like Mat Best, doesn't mean they're not a veteran. Even if someone walks around in a perfectly squared away uniform, it doesn't mean they're a veteran. Stolen Valor dirtbags have probably figured out how to use Google.
If you're uncertain whether someone is really in the military or faking it, talk to them. Google will only help them out so far. Pull them aside and ask them a few questions, calm and collected, so they're off-guard. Bear in mind, if they fail a question, they may still have served. Traumatic brain injury and dementia are common among veterans. If you're giving hell to the guy who can barely remember his daughter's name because of an IED in Iraq - you are the dirtbag.
The trick is to catch them playing along with a lie you made up. Praise something that doesn't exist and if they latch on hoping to get your approval, they're full of sh*t. Add in minute details that should set off red flags if they don't look at you like you're crazy. From there, it's up to you. I, personally, recommend just shaming them into going back home and changing out of the uniform of good men and women. You do whatever you see fit.
"That's impressive, I heard about the serious fighting in Atropia, Iraq. Were you there?"
For some reason, no one ever pretends to be a part of the 97% of the military that are POGs. Stolen valor dirtbags always go big. If you make up some random place that sounds vaguely foreign in Iraq or Afghanistan, they won't know that the place doesn't exist.
"How long did it take you to make insert a rank not indicated by their uniform?"
Memorizing very important details is hard for dirtbags. Specifically, details like believing you can make E-7 in three years. Added bonus if they don't correct you on saying the rank incorrectly.
"Did you ever serve with my buddy Wagner? Man, I can't remember what that dude kept going on about loving..."
If there's one thing you can always count on is civilians not truly understanding the real size and scope of the military. With over 2.2 million troops in the United States Armed Forces, there's no possible way to know every single person serving.
"Oh nice! What was basic training/boot camp like? Were the Drill Sergeants/Instructors mean?"
Soldiers do not go through boot camp. Marines do not go through basic training. To civilians, they're used interchangeably.
If you intentionally mix them up, and they don't politely correct you or immediately look at you like you're an idiot, you got 'em.
If they're in a dress uniform: "That ribbon is nice. Did you get it for [whatever]?"
According to basic human psychology, liars always elaborate their stories to try and make their story seem more believable. If you point higher up on the ribbon rack, those can be awarded for some insane things. But it's the lesser awards that are basically handed out for not messing up anything. When you point to, say, the Global War on Terrorism Service Medal and they say it's for saving their platoon: laugh.