Dear Mistress of Snarktitude --
I've got one for ya.
So say an agent CALLS (yes CALLS) and says they want to respresent your beloved cherished novel --
and you WANT to say yes, but then Miss Snark's holy words reverberate in your ears -- check with other clients first.
Now how do you gracefully do this? Do you hunt them down yourself? Ask for a list of "references." Trust that this agent has sold many a book cause you see them listed at Agent Query, on the agency website or on Publisher's Marketplace?
Been querying agents you don't know, huh??
You know my position on that; all together now: do not do that.
By "know" I mean not that you have tea and crumpets on the lanai regularly, but that you've done some basic research like finding out who the agent represents and what they sold.
When an agent offers you representation you say very nicely "I'm thrilled. May I contact some of your clients to make sure you're not a scuzzbucket". I give out client emails at that point (but not before) and only of those clients who've said "sure" when I've asked if they'll be willing to speak to prospects about the Wonders of Snarkistan.
Listings at AgentQuery and Publishers Marketplace are just that: listings. You want to be in communication with folks who've actually worked with the agent.
ALL reputable agents agree to fork over this info. NO reputable agent refuses it. They may not give you Thomas Pynchon's email, or their really famous clients, but there should be contact info for legitimate clients with books that have sold. Do NOT accept any equivocating on this subject.