It's worse than that, how do you know it's a good agent without completing the transaction? It seems you pretty much need trusted word of mouth referral.
It's even worse! Even after you have completed the transaction, you don't have the counter-factual and never will: Could someone else have done it better/faster/for more?
I would think a referral is only really excellent if from a person who had to do many transactions recently, and sampled at least 2+ realtors. And that's something that's quite hard to come by.
It's just like any hiring process: get referrals from people you trust, look at prior work (previous purchases/sales in this case), learn the right questions to ask when interviewing.
