Tonight with your 13 to 17 year old
The prompt
“Is there a joke or a bit that's become a running gag with certain friends?”
What to listen for
If they light up telling it, even if you don't fully get the joke, that's the point, inside jokes are a sign of real, easy belonging with specific people, and being asked about it tells them their world outside the house matters to you.
Notice which friends come up; that's useful context for who's currently central in their social life, without you having to ask directly "who are your friends now."
If they say "you wouldn't get it" and leave it there, that's fine, let them keep some things just theirs; you don't need the punchline to have made the connection.
What this tells you
This tells you where they currently feel easy, unguarded belonging with peers, which is a great sign of healthy friendship.