I believe that you may not evaluate one's skill in a language, except as others already said, by the knowledge of tools and/or frameworks, plus the subtiles of the language itself.
I think this is a very subjective question, as ones would say that one needs to know this, can do that, and others would say the total opposite right after.
In anyway, what I think may count the most is what skills one has for the job you need to get done. Then, perhaps reading his résumé, notice the kind of project this one has worked on, and for how long he's been doing what he was tasked to. This could give you a good approximative idea of his skill.
But once again, perhaps looking to the technical environment the man/woman has worked into, what other technologies he had to deal with etc. I guess you get my point there.