In almost all cases, a person’s sexuality is strictly her own business. Attempting to "figure her out" just to satisfy your curiosity is unethical, and could even put your friend at risk of homophobic harassment. However, if your friend is crushing on you or vice versa, you may need to address it. Instead of trying to settle the question of your friend's identity, figure out what your goal is. Are you trying to defuse the situation and just be friends? Or are you trying to turn the friendship into something more? Whichever side of this you're coming from, it takes a careful approach to avoid damaging the friendship.