Five or more years is significant for this type of business. A business that is carefully managed and provides a good value for its' clients will stand the test of time and survive even when the biggest and most extravagant studios fail. It is generally in the best interest of the client to get a reference. Feedback from other clients can be very enlightening. Ask other artists how they like the product, the engineer, and the sound.
References can give valuable insight into the efficiency of the studio.
Also ask if they would go back to the studio again. The performance room needs to be big enough to have all the musicians perform together at one time. An entire band needs to be tracked at the same time, however, because of the volume additional isolation booths are required. The acoustics in the performance room can also be a consideration depending on the musical style. A large control room makes working with the engineer more comfortable.