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.
Question: How long did Baby serve time in prison?
Answer: Baby is sentenced to twenty five years in prison, and is given a parole hearing after serving five years of the sentence. Baby is then released after the parole hearing, spending five years in prison.
Five years is correct - for one interpretation of the ending. Another possibility is that the ending we see is a fantasy and Baby served the full sentence. Some fans have this theory because Baby and Debora have not aged, and the car and her outfit are almost dreamlike in their presentation.
Answer: He has a US Marines bumper sticker on the back window of his truck.