Gautama Buddha was a special case of someone in whose past lives there was endless awakening. It was for this reason that he was able to be born with very little karma left, making him able to get enlightenment through his individual practice of asceticism. So he's the one who became a Buddha for the very first time from being a Bodhisattva. But keep in mind that his practice was very different from anyone around him, and there was no one who practiced the meditation of the day who was able to be Enlightened like he was.

With that foundation, I can answer your question a little more directly by telling you that he was able to realize Buddha's teaching through the world itself, after he opened his own eyes. (Buddha's teaching means how the world is operated, so everything is its evidence and can be found through any examples)

If you have any doubts please keep asking questions :) This can be dangerous territory if mistreated