1. Are they a licensed tax professional? Enrolled Agent? Certified Public Accountant? Tax Attorney?
2. Make sure they are NOT strictly a salesperson - these are pretty common with the companies that advertise on TV - these people have no license to keep them honest.
3. Do they have at least 5 years of proven practice on IRS/State collection cases?
4. Do they charge a flat fee? If they take monthly payments over a long period of time, that can be a deal breaker: they may use your unpaid fees as a reason to NOT finish your case.