In my story, a French soldier from WW1 died in a horrible way, and became a ghost. As such, he's still wearing the same military uniform as he died, the same equipment, etc. He can react to his environment, and talk to people, but he doesn't realise that he's dead, and that he's not between 1914 and 1918 any more.
Now, in late November 1944, he has joined the American army heading to Germany to end the war.
Beside the fact that he cannot die or get wounded, how could one guess that he isn't a soldier from this period? Or at least, what could make someone suspicious about his true identity?
Edit : Thank you for your numerous and interesting answers. First, I'm a beginner writer, and this is my first question on this site. Also, I'm French, and as we've established, French people are very bad at English, and I'm sadly no exception.
My story follows the American army, and not the strange French soldier. The point is to fool the reader about the ghost's true identity. Then, is there a way to make the misunderstanding the longest ? What are the hints that just make the character appear weird, and what constitute a blatant proof of his weird state ? Also, I said that the soldier joined the American army, but let's say he just met them, and is trying very hard to join them. Is it too much of a stretch for the G.I. to accept him, whatever he says ?