how can you see if someone really is in the army?

[CharlieFoxtrot]

You can't check some master list online of who is actually in the Army. There is no such thing.
But you can check a few things:
Be upfront. Tell him there are a lot of military romance scammers out there and you need him to do a few things. If he is legit, he will not be offended.
1. Ask him to send you an email from his official US Army email account that ends in .mil
2. Scammers usually say they are deployed. If he says he's deployed, ask for his APO mailing address so you can send him a care package. Tell him to write you back so you can see the military postmark.
3. Have a Skype video chat with him.
A scammer will tell you all those things aren't possible for some BS reason like "mail is too dangerous" or "I have a super high clearance and am not allowed".
A real Soldier can do all those things.