This is a question I've pondered often.. and stumped many a roommate with. Consider this: we can not know if we are making a true free choice or simply doing what we must do. To say it another way, human beings, no matter how much we'd like to think we can, can't know if we have free will or not -- a thought that we consider spontanious could be totally and completely predictiable if we could see/understand the inner workings of our brains.

Simply put, the best you can ever do.. is pick one option at random and cobble up whatever seems sound evidence at the time to support yourself.