As our business has grown, it seems like it is time to "get
professional" and bring in a custodial service. We're in a building
with common restrooms, so most of the work involves emptying trash,
vacuuming, cleaning the mini-kitchen area, etc., plus being reliable
enough that our computers stay put and (even more important) our
customer records and work doesn't leak out. How do you go about
vetting these firms, and what if anything should I watch out for? In
case it matters, our firm produces newsletters and promotional flyers
-- nothing with chemicals or anything sensitive like that.

It'll cost you a lot more than $3 to hire a custodian and it would
take a Google Answers Researcher a lot more time than $3 is worth to
research this question.