Ask the studio's you call if you can visit with them prior to making a decision. I think you'll find that, although they are very busy, they will be happy to show you around and maybe even boast a little about their equipment and previous clientele. That is a good place to start and ask if you can hear some of their work as well. Get something in writing as far as rates go to compare as you continue the quest for the perfect studio for you. Most studios offer block times, which is a predetermined amount of time that you can pay for in advance.
Studios like to have time booked in advance and offer discounts for pre-purchasing blocks.
Some even offer discounts for cash. Some studios will charge more for an engineer and even offer a producer for an hourly rate. There will be extra charges for media too. One will usually do the job. Some studios offer in house mastering, artwork, and even bulk CD orders for a price.