How can I find out if a company is a guild signatory or not?

[Mac H.]

One thing to remember - an agency who is a guild signatory is 100% permitted to rip you off.

The rule is that they are not permitted to rip guild members off.
However, as the guild is technically a labor union under US law - they are very limited in their rules - they are not actually permitted to make rules about how they treat non-guild members. (Production companies are a different kettle of fish because they are employers. However an agency isn't an employer. A labor union isn't allowed to make rules about how non-employers treat non-union members - it is outside of their charter.)

I was surprised at this as well, and so confirmed it with the WGA itself.

Mac