What is their experience with solving IRS problems? How long have they been operating in this business? Of course the longer the better. A CPA or a lawyer may just be involved in tax help on the side and may not have the knowledge and dedication to deal with intrinsic of IRS negotiations.
How many tax lawyers do they employ? Some firms are just CPAs, while others hire only former IRS agents. There are also typical law companies with only a few people specializing in answering your tax help questions. Firms that do not deal with tax problems on a daily basis will not be aware of all the loopholes and pitfalls of this legal area. You should find a company with a certified tax resolution specialist. There people are obliged to pass a special exam and must have a considerable number of years of experience and continuous professional development. You will not find more that two hundred of those in the country, so look carefully.
What is their success rate with tax cases? Do not let yourself be fooled with some generic figure here. Ask about the success in solving tax help questions and cases similar to the one you are having. Do not count on perfect results, though. For instance, in the majority of cases the Offer in Compromise is a starting offer. Only as much as 2% are instantly approved by the IRS. You should be more interested in the number of cases similar to yours. What is the total dollar amount negotiated in settlements divided by total dollars in tax, interest and penalties owed? To put it simply, how much money were the tax attorneys able to save their clients?
Finally, you tax help questions should include one about offered guarantees. If they do provide one, look for a different company, as it is impossible to guarantee anything in this business.