Good mentors are hard to find, but when you happen to come across one this is how you can tell:
He has respect for your dream.
He respects the things you are passionate about.
He does not try to force his own agenda on you.
He does not ridicule your inexperience and errors.
When you mess up, he corrects you with respect.
His intention is to see you become great, not to use you for his own greatness.
He can become hard, but with the intention of getting the best out of you.
He is ahead of you in the area you desire help in. Your mentor must be at least a step ahead of you, else you have nothing to learn from him.