Ask them

Most foolproof way I can think of

The checking for a ring is somewhat helpful ring+not single

Unfortunately no ring does not equal single

Ask them.

If they give you their home phone number, not their cell.

Although for some people cell phone = home phone, so even that one's dicey.

In this day and age, there are no sure signs. As others have said, you need to ask. If not them, then someone who knows them.

Mendalla

Ask them

Relationship status on Facebook -- but then not everyone has Facebook and not everyone posts their relationship status there...

If she wants to stay the night after having sex with you...

Some people have more than one so that's not a guarantee either.

How about

Ask them out?

Has to be ask them.  I always give my cell number out because I am almost never home.

And not everyone is totally honest about it, esp. if they know/think their spouse isn't online.

Mendalla

Since I think the question is more "Are you single and uninvolved?" i think the only way to find out would be to ask them.  Not "are you married?" but 'are you in a relationship?'   That, I think , would cover not only married, but engaged, common law, or committed friends.    Of course they might lie.  They might be unattached but just not interested: they might be married and out for the night.   Without being too suspicious (which might ruin your chances) there will soon be signs.   Like 'here is my phone number.  If a man answers hang up.'   unexplained avoidance of certain places.  Always wanting to meet in out-of-the way places.   Nervousness - looking over her shoulder, checking out a room before she enters and then sometimes refusing to go in.    And dozens more.   Or asking somebody who has known her for a long time.  

A more delecate question might be 'are you interested in the opposite sex?'

It depends on the culture. In my wife's home country it's if they celebrate black day.

MorningCalm - I was talking to a Korean acquaintence recently about Valentines Day. She was very impressed that I knew about black day and white day! I have you to thank for that.

Hey, you're welcome somegal. Which reminds me, I'd better buy something soon for white day. The tradition is candy wrapped in white paper. I might go above that, though. She's certainly worth it.

If you are talking with someone and her friends find an excuse to leave you alone with her - it's a good bet she's single.

I have instinctively done that myslf.

If they ask you out odds are they are single (or a lout!)