Specify the reference you want
If you leave the choice open, you will receive the provider's three most enthusiastic contacts, which tells you they have at least three. Instead, ask for a site of a similar type and comparable size, serviced continuously for at least twelve months. That constraint does most of the work. A provider who cannot produce one is not necessarily unsuitable, but the reason becomes an interesting part of the conversation.
Be reasonable about what you request. Providers cannot hand over client details freely and many organisations do not permit their staff to act as referees. A provider offering to arrange a call rather than supplying a phone number is behaving properly. What you should not accept is a written testimonial with no contactable person behind it, since that cannot be tested in any way, because nothing behind it can be verified by you or anyone else.
