Unfortunately, there are no standards for mediators, and in most states, anyone can hang a shingle and call himself or herself a mediator. Your state or city might have an organization that admits mediators based on training and accumulated experience, and if so, look there for a list of references. Your local bar association might be able to give you the names of reputable mediators. Word of mouth is a good way to locate an appropriate professional as well.
The mediator should at least have a degree in social work, counseling, or psychology. It is helpful, but not imperative, that the mediator have a law degree as well. Even without a law degree, however, the mediator should be completely familiar with the divorce laws of your state.
How can you find out whether the mediator knows the law? Ask how long he has been doing mediation, how many cases he has handled, and how he keeps up with new developments in the law. The prospective mediator should be able to answer your questions without sounding defensive.
Find out what and how the mediator charges before you hire one. You and your spouse should agree on how the mediator's fees will be paid. Will you each pay half the fee at the end of each session, or will one of you pay all of it and be reimbursed when the case is over and the assets are divided? Will one of you foot the bill with no reimbursement? If you cannot work that out with the mediator's help, mediation might not be for you.
Finally, ask for references—and call those references. The mediator might not be willing to give you the names of clients (and should not do so without the clients' prior consent), but the mediator should be able to give you the names of attorneys familiar with his or her work. After all, the mediator should be sending couples to attorneys to review any agreement before it is signed. If the mediator doesn't know any attorneys or can't give you any names, interview someone else. Sometimes, it is useful to engage in a dual mediation process where there are two mediators, one a lawyer and one a psychologist. This raises the cost, but in the right case, it might be worth it.
Before choosing your mediator, fill out the candidate's credentials on the mediator checklist to decide whether a given mediator is for you.