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I've struck up a friendship with a co-worker. I'm pretty sure he's gay (almost positive), but I doubt he knows that I know. He's been keen on meeting me outside of work - we've met for dinner. Is he coming on to me? Is it a good idea to give in and see where this goes? We are equal when it comes to our jobs.
Never assume that a friendly guy is gay or wants in your pants/bed whatever.
The first thing you need to evaluate is how being outted at work will go over. If this is a no go situation, then you are going to have to seriously get to know this guy's political views on such topics as Apple's New CEO coming out, Or Russia's taking down the Apple Statue, or the impending new gay marriage in X state.
All of these are semi-neutral topics which shouldn't arose too much suspicion since they are currently in the news.
Once establishing if he has a tolerant or intolerant view on LGBT issues you can proceed from there. Asking him about his 'ethics' on outing gay people at work helps, if he tells you such is deplorable, unacceptable or something like that then consider yourself safe to come out to him.
Its still a 'bad idear' to ask a guy if they are gay. The habit of haters is to get a guy to admit they are gay then proceed to bash. So even under the best of circumstances its best to not ask, but to inform of your stance and let them decide when (if) they want to come out.
I would also suggest to strive for a friendship over a relationship/dating/FWB type thing.
I have mixed feelings on dating a person from work. my first lover and I met and worked together for over 2 years as friends first, then as live in lovers. mind the circumstance wasn't complicated by job competition nor a need to hide ourselves as we were, often the only two people on the job site the hours we worked.
So depending on your situation if anything comes out of this it may not have that big of an impact on your job.