You can estimate the literacy of a forum by choosing "annotate results with reading levels", then doing a search for site:example.com. It gives a breakdown of the fraction of pages at each reading level. Far from perfect, but interesting nevertheless.
If I understand it right, this does not really measure literacy, but the proportion of complicated words used. Mathworld if a site about math, and math uses a lot of special terms, so of course it has a high reading level.
