Toni - A successful advisor will have multiple satisfied clients, and will not hesitate to provide you references if you ask.
I've found it's generally a matter of expectations. Quality advisors are generally careful about which clients they accept. A client chasing `performance' is not one that will be happy with any advisor for very long; a disappointment is bound to happen. Likewise, a client seeking a long term prudent and disciplined investment process operating according to a plan is likely to be disappointed if s/he has an advisor who's always coming up with a new investment idea.
When the client and advisor are 'in synch', the odds of disappointment go down dramatically. I think the referral factor is one component - remember, even Bernie Madoff had glowing referrals and all from people chasing perfomance - but, being aligned on process may be just as important.
If they aren't transferring their assets away, are providing referrals, answer your correspondence, then it is safe to say they are happy. Especially if they are referring. You wouldn't refer to someone you don't like