I've heard a good tool to use for a person starting out to answer that question is to do a simple exercise. Clap your hands as if applauding for 10-15 seconds. Have someone watch you closely when you do it.

If you are lefthanded and you are observed moving both hands into each other, or the right hand into the stationary left hand, you should have no problem learning a right handed guitar.

I'm no expert, heck, I first started playing myself recently, but a lot of people seem to think that lefties who play right have it "easier" with guitars when they really don't. Sure, there is some advantage with making chords with your dominant hand when starting out, but the heart and soul of guitar is from your PICKING hand.

That fact really seems that the dominant hand would be the one you would want picking.

Don't just swap the strings around though. That will only hurt you in the long run.

Righties have advantages though, tabs are typically written for righties, so we don't have to "fix" them. Also, the gear is much more common, so we can get frequently get it at better prices, and it has greater resale value.

That being said, go with what feels natural. Don't just "hand them a guitar and observe how they hold it" as often times a person will just mirror what someone else does if they don't know what to do. Have them try it both ways, the earlier in their playing the better!