Is hubby cheating

I think you are making this up.

Keep stum and gather evidence. Keep a diary. If you merely ask about a couple of stray hairs (he will deny anyway) you have nothing else to fall back on. You need to be able to present several incidences. Reduces the ability to wriggle out of the situation. (and keep the hairs in a plastic bag!!)

I fear I know the outcome. Good luck.

"Our sex life had dwindled over the years "
And does this upset you, or are you happy with the separate rooms and the lack of sex?
If you want sex, then talk it out with him, or go to Relate. But be prepared for not liking the outcome.
If you are happy, then leave it alone. And ignore the porn, that would be his business.

Yes I agree with mumofmadboys and don't jump to any conclusions. I'm sorry you are having these concerns though,

Retired at 53 , he must be bored ,

Well there’s really not a lot to go on He might have been watching porn to try and liven up his flagging sex drive perhaps a shaggy black dog visited him while he was chopping wood
As you don’t have a sex life and sleep in Different rooms maybe instead of getting angry and imagining things you need to sit down and see what you both want in your older age

Tell him you found two long black hairs in the bath and you wonder where they came from in a relaxed way and see his reaction.

I think the only thing you can do is ask him.