Difficulty socialising with boys as a gay person

I'm open about my sexuality when it comes to my peers and people my age, I often go out on the weekends to pubs to meet and socialise with other people. In my circle of friends I'm often known as the nice guy, the person who knows how to be a friend, that traditional stereotypical boy-next-door-who-will-listen-to-all-your-problems.

The main problem is that whenever I find a boy that momentarily makes my heart skip a beat or two... which is... pretty much any average boy that isn't a bitch. I have difficulty flirting or talking to them. The problem is I don't know if they are gay or not. And I can't go up to them and ask them if they are gay because that could be considered as rude. I've tried it before and often I've been given that judgemental stare. I don't know what to do, all my friends are pretty much coupled with someone else. I want to know how it feels to have someone take an interest in you. Valentines Day is coming soon, even though I'm pretty sure I will spend it alone again, I really don't want to this time.

So how do I socialise with boys beyond the normal levels of just sharing pints? How do I flirt with right boys? How do I know their orientation? I want to build intimate relationships that last.