Here are three tricky ways to figure out how you will most likely feel most comfortable standing on your board:
ONE- go get a ball or something like that, and sit it on the ground in front of you. Now kick it. Whichever foot you kicked it with will likely be your back foot. You want the balancing foot in the front, and the kicking foot in the back.
TWO- go to a staircase, and walk up it. What foot did you use to step up the FIRST step? That's likely your back foot.
THREE- this one's hard, because once you read it you'll know the trick. So just read this first spot and then stop when I tell you to stop. Go find someone, stand with both your feet close together, and ask them to shove you from behind. Go do this now -- STOP READING! Ok, so hopefully you didn't just fall over. You should have caught yourself with one foot. The foot you used to catch yourself is likely the foot you'd put in back.
Just like most people are right handed, most people are regular footed. That's why it's called regular. Just remember that there's no RIGHT way to do it. If all of these tricks tell you that you are regular, but you just like riding goofy, then ride goofy!