Question: When the Marine in the Dodge Ram pickup interrupts the armoured car robbery, he is wearing civilian clothes, not a uniform. How do they know he is a Marine?
Answer: They don't know he is a Marine per se. They are being facetious by calling him a marine based on his gung ho attitude, the fact he has some type of training, and his determination to stop the robbers. Also, his style of dress seems stereo-typically military even though he is wearing civilian clothes.
He has a little marine corps logo sticker on the front window (where a break tag would go) and a giant USMC sticker on the back window. As someone who spent the vast majority of her life around Marines and military lifestyle, things like that pop out like a strippers nipples.
Answer: He has a US Marines bumper sticker on the back window of his truck.