Play in another GM’s game. Or co-GM a game with another GM. Take notes on what you liked and disliked about the experience. Set up time to discuss their game with them and get feedback. Walk through an encounter or two with them from your planned game. Share a speech about what you noticed while playing now that you’re working towards becoming a GM. What will you be stealing for your own GM style? How did playing/collaborating in someone else’s game change what you were planning? What feedback did they provide you on your original ideas?