Would you tell a man he's got a urine stain on his trousers?

It’s something I am very aware of get sick of informing the person involved and I get told off for making it an issue!

He certainly needs a good (kind) telling then.
He'll end up smelling.

I could, except I don't see them at their home so they are not able to when I see them.

It's definitely not water splashes as it's a round mark in a certain place. As I mentioned it is sometimes brown from repeated markings when the same trousers must have been worn several times.

It isn't my husband, but I asked his advice and he was quite defensive about older men urinating, saying "quite often you think you have finished and then more trickles out". I said surely in that instance it's good to put some tissue in your pants or a pad?
My husband only wears dark trousers (thankfully), so even if HE does it, it's not obvious. Light coloured trousers are not kind to their wearers.

You could suggest he put on a clean pair.

If he is “in your life” - yes.
If just someone you meet occasionally, no.
Is there no mutual friend who could help out?

Even if they fume at having to change their trousers it’s important to get this sorted.

I would sure tell DP if that happened.
Could it be water splashes when he washes his hands tho?

It depends who they are, really.

On balance, though, I'd tell them.