What's the single best question to ask?
'What did you change on a client's site last month?' Not what they reported — what they changed. Vagueness here is the whole answer.
Someone doing real work can describe it immediately and specifically: these titles rewritten, these four pages merged, this schema added, this category changed. Someone producing reports will talk about strategy, momentum and algorithm updates.
It's a hard question to prepare for and an easy one to ask, which is what makes it the most useful thing you can do in a sales call.