If you complain because a textbook lacks footnotes and sources . . .
If you read through the latest National Geographic stuff catalogue and catalogue the errors (“No, Spanish people were not in Peru in the 15th century!”)
If friends refuse to watch historical movies with you because you critique technical details (Also applies to pilots and aviation movies)
If you fight the urge to add more detail to a tour-guide’s narrative. Or to correct the guide. Or to correct guidebooks
If everyone around you asks you about history stuff, “Because [name] will know” and you do . . .
If it seems perfectly natural to list “17th Century Warfare” as one of your hobbies (and you are not a reenactor).
You might be a history nerd. Or a professional historian. Or yes.