If you are in the market for Drupal development, you may feel like you are trying to pick a car mechanic without knowing anything about cars. Like picking a mechanic, you often have to go on how you feel about the vendor. You should listen to what they say, and how they say it - carefully considering how that makes you feel.
Do your clients love you with such a furious passion that it borders on problematic? Do you find new customers calling you and saying things like, "I must have your services! My friend has been bugging me all week to call you. Seriously, I just want him to stop camping out on my lawn. It's scaring the children."