What sort of studio do they have?
As I rambled on about before, home studios are popping up all over the place. Be sure to inquire about what kind of gear your radio or television voice talent is using. If they go to a professional studio, great....but don't count on getting your stuff voiced with a modicum of expediency - they'll have to check for avails, if you know what I mean.
If they have a home studio, it doesn't hurt to have your producer or engineer call the voice talent up and ask a few questions. They should be using a high quality, large diaphragm condensor microphone, a good pre-amp, mixer, phone patch/ISDN, sound card and sound proof booth. Your voice talent service should also have high speed internet access and a dial-up service as a back up (DSL and Cable can sometimes go down). Pay attention to the way they answer questions about studio gear. If the voice talent is running a quality home studio, then answers should be quick and informed. If they stumble and have to run around getting names and numbers about their own gear.....then beware the rookie. Anyway you slice it, critically listening to their audio is the best guage overall. If the product sounds clean, free of noise and other anomolies....then the voice talent probably has some good voice over gear and knows how to use it.  
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