Determine if an index is a B-tree, GIST, or something else?

Is there any way to determine, by querying pg_index and other pg_* tables, whether an index was created as `USING something`? I've already got a big query joining pg_class, pg_index, etc. to pull out various attributes about the indexes in my database, and I'd like to include whether it's a GIST index, a B-Tree, or whatever was in the USING clause when the index was created.

I see that I can join with the pg_opclass table, but that has 100+ rows, and I'm not sure how to tell what is a B-Tree and what isn't. Any advice?

Thanks,
Paul