I play guitar right handed because that's what my first guitar was. Before that, though, I played air guitar left-handed and always thought that was the right hand method (since the right hand is moving over the frets, and that seemed like the brunt of the work).
I started writing left handed when I was a kid. In 2nd grade, I was tired of bumping elbows so I just switched to my right.
I throw a baseball, football, whatever with either hand. I prefer outfield left handed and infield right handed for baseball.
I bowl with either hand but I don't like to own several bowling balls, so I switched to left to maintain a more consistent lane condition since less people use that side.
I usually shoot right handed because most guns prefer it. But I switch it around every once in a while.
I can learn how to do something with either hand and don't have a specific preference. However, if I have to learn how to do something - such as play guitar, or any kind of task that requires practice to get good at - if I don't practice with both hands at the same time, then I lean to right or left hand depending on whichever I start with and works best.
Some times I have no idea which is considered left handed or right. Sweeping with a broom, which hand holds the pisser steady when urinating, carrying stuff while walking, clasping my fingers together to find the dominant hand, skateboarding, etc...
I prefer to skate goofy foot, but I had no idea that was unusual.
So in the end, I am most likely ambidextrous but with a tendency to left hand dominance. Hard to say. I write with my right hand, do most others things left handed. Reason being, though, is that the left hand can often be an advantage in many situations.
As for playing guitar... I am quite proficient as a right handed player, but I always think that if I committed myself to playing left handed I would be a little bit better because I think I could fret notes faster with the twiddly-diddlys if I was using my right hand for that. I just don't want to relearn how to play... it might only take a couple months, but getting new callouses, wanting to restring guitars, trying to find good left handed guitars... pain in the ass.
I know a left handed guitarist and I asked him about whether it felt right to play right handed. He said he used a right handed guitar for almost two years and played like CRAP. He decided to buy a new guitar, and within a few months playing left handed he was way better than he ever was using his right. For me, I imagined myself as a left handed player but because I was given a right handed guitar to play, I just decided to learn with that hand. It was no big deal.