Written in first person perspective for clarity. The list ranges from novice to expert in that order. Exceptions exist, and this is just a list of common symptoms, not a roadmap.
The thing tends to excite you a tad bit more than it does others.
Reading / learning about this thing amuses you.
After a while, you tend to see tiny details others usually ignore.
Creating something yourself related to that thing keeps you in the zone.
You have fun being in that zone.
You can anytime pick creating / learning this thing over a dinner hangout with friends.
You start appreciating what others have done in this field in a much deeper way. That is, you’ve come to see the true genius of someone’s work. Not because the rest of the community appreciates his / her work.
You create stuff, that folks in that field find interesting and useful.