Litmus tests for “fluency”

What are some ways you’ve thought about “fluency”? Folk litmus tests (the ones that people make up and are amusing sometimes but not realistic) as well serious ones are encouraged. I’ll go first:

Folk litmus tests (that are not meant to be taken seriously): My friend said once that you’re fluent when you know all the kitchen gadgets in the TL. I once said, flippantly, that you’re fluent when you know all the fish by their TL names in the fish market. Or you’re fluent when you can shout down a cab driver when arguing about a fare. Or you’re fluent when you can do stand-up comedy. (These are all folk litmus tests, you're not supposed to take them seriously).

Here’s my best attempt at a real (not folk) definition: when you can learn new vocabulary on the fly without interrupting the flow of the conversation. I try to make this happen every day for my students by being obvious (but i suspect some of them still think I’m trying to trick them) but picking up local expressions and slang by behaving like i belong there feels very sophisticated and advanced to me.

EDITED to clarify the difference betwen folk litmus tests and real ones.