There's a tip on this page: 5. Setting up your company :: Freelance UK about asking for hidden charges, exit fees for instance. Also some will claim to protect you from IR35, which isn't how it works at all - the Revenue would only judge this on your contract and how you actually go about your work with the client.
Some do have a better reputation than others - Google a few and see what comes up on other forums as well as this one.
Best of luck