The quality of a datacenter has less to do with the equipment (although thats important), and more to do with who designed and is running the equipment.
Most of the datacenter outages I have been a part of in one way or another (Customer, or Provider) have been caused by:
As a normal customer, there is no way to know if any of these problems exist. The solution? Ask for references that utilize that datacenter. Make sure they don't give you a customer that utilizes another data center from the same provider. Data center design varies greatly, even across the same provider. Ask that reference how long they have been there, how many problems they have had, and the companies response to those issues. Look for a customer with a long history in that data center (3+ years, 5 would be better).
Don't rule out a data center because they had an outage. Outages will happen, no matter how redundant their systems are. Their response to it is very important. If you find out about a previous outage, ask to see the root cause analysis they provided their customers. If they can't or won't produce it, even under NDA, then walk away.