What would qualify someone to teach meditation or share other knowledge gained from the study of Buddhism or zen or otherwise? How do you know your experience or wisdom is enough and is beneficial to pass on to others?

You can start to teach the form of Zazen anytime. Yet for being able to teach the mind of zazen, you need to follow a true teacher, and you need to receive permission from that teacher to be allowed to teach the mind of zen. Only then can the teacher make sure that your mind is the same as the mind of the Buddha. The form of zazen can be understood by anyone and you can correct others, yet the mind cannot be perceived from the outside, so you need to have approval of a teacher yourself.