I've often considered a self-assessment system where the sitter is prompted with a series of terms from the topic at hand, and asked to rate their understanding on a scale of 0-5, with 0 being "I've never heard of this concept", and 5 being "I could build one of these myself from scratch".

The terms are provided in a random order, and include red-herring terms that have nothing to do with the topic at hand, but sound plausible. Whoever provides the dictionary of terms should have some idea of the relative difficulty of each term, but you could refine it further and calibrate it against a sample of known diverse users, (novices, high-schoolers, undergrads, etc.)

When someone sits the test, you report their overall score relative to your calibrated sitters ("You scored 76, which puts you at undergrad level"), but you also report something like the Spearman rank coefficient of their answers against the difficulty of the terms. This provides a consistency check for their answers. If they frequently claim greater understanding of advanced concepts than basic ones, their understanding of the topic is almost certainly off-kilter (or they're lying). The presence of red-herring terms (which should all have canonical score of 0) means the rank coefficient consistency check is still meaningful for domain experts or people hitting the same value for every term.

Actually, this seems like a very good learning-a-new-web-framework dev project. I might give this a go.