Ask them their army number, they should be able to rattle it off. Ask them their unit, they should be able to name company, battalion & regiment. Look at the Army Rumour Service, they may be able to suggest sources of information about previous service.
Thanks - yes they claim (and have done publicly) to have served and I don't believe them. Pretty sure they would never have passed the medical. I can't ask them directly, I'll check out the rumour service. Thanks again
From experience there is quite a few people who do this. I've met someone who said he was in the RAF, "where were you stationed and when I asked" hoping to make some conversation "oh, only RAF Swinderby, I left after 5 weeks because I didn't like basic training"
To him though, he had served in the RAF. Strange.
To answer the OP, you need his service number to be sure of anything.
You could google for the regimental/corps association and see if he's mentioned on any of their events, or if they have a 'find a friend' type function, try enquiring (they can be a bit cagey about giving out people's details, but often agree to forward plausible reunion requests)
People might be cagey about service number if they use it as a password, but it's something everyone can rattle off.
Does he have friends/acquaintances from military days - people don't always stay in close touch, but they know each other.
Are there group pictures of him in uniform, or in an identifiable military sports team?
Are there bits of (usually very random) memorabilia in his pad?
I had someone tell me they served 6 months in jail for being involved in a crime for the IRA. Found out that he couldn't possibly have as minimum sentence for membership is 6 years. The bollix. A lot of Irish men try to let on they were in the IRA the fucking eejits as they think it makes them look like big bad boys not to be messed with or something.
You only have to have served for a day to be entitled to help from any of the armed forces benevolent funds! Many people claim to have served and never passed out of basic training. Being former serving RN this does infuriate me. Especially is they are trying to gain from claiming to be a veteran.