This is what I always wonder in the back of my head as well. Take passive vocabulary size - it should be fairly easy to show people who claim to speak a given l

This is what I always wonder in the back of my head as well. Take passive vocabulary size - it should be fairly easy to show people who claim to speak a given language a random selection of words in various frequency bands, and ask them either what they mean or what other words out of a list are closest to it semantically, to estimate their actual vocabulary size. If you don't think showing isolated words is an adequate metric of someone's true gloss skill, you can show words embedded in sentences. But we never even hear about these kinds of estimates in these stories, let alone anything more in depth.