...you can name all of the characters and their respective spell cards and difficulties from games 10 through 12, are making a fangame of them, draw fanart, create a character in your comic based on any character in the Touhou series, play the same stages over and over again just to maximize your strategy, can distinguish all of your weapon's special abilities, their respective youkai (Subterranean), can 1CC 10-12 and also beat the extra, 1CC Perfect Cherry Blossom and spend hours attempting to beat Chen in the Extra, and finally, you know you're addicted to Touhou when you go out of your way to list all of your connections to it and said list gravely outshadows what you thought it would be.
I played it for the first time three weeks ago. And I'm already way better than the guy who gave me his copy.