When your friends know that they should not say anything about languages/linguistics unless they want to be involved in a very long discussion.
When you have a friend who is also a language nerd and you agree not to talk about languages for a change, only to find yourself discussing Hebrew verbs 5 minutes later.
When you throw in random comments/questions about the speaker's grammar/pronunciation in a conversation ("Did you just use a plural as the first part of a compound? Awesome! Er, what was the question again?")
When you start analyzing people's slips of the tongue to see if they fit with Levelt's model of speech production.
When a friend says you look like you're in love and it turns out it's just because you've found that awesome Portuguese textbook at the second-hand bookstore.
When you play Glossotechnia
in a pub with a bunch of Dutch and German people while a soccer match between Germany and the Netherlands is on TV, totally ignoring the match but getting excited about somebody drawing an exotic phoneme.
When hanging out with some 70 linguists at a student's conference is your idea of one of the greatest weekends ever (including making up satirical linguistic theories at a party and leaving notes for each other in IPA in the local dialect).
When your favourite jokes are not considered funny by non-linguistically inclined people. [A verb walks into a bar and sees a hot noun sitting at a table. The verb walks over. "Hey baby, wanna conjugate?" - "No", says the noun, "I decline."]