This all comes down to two things. Experience and integrity. The vendor you choose should have testimonials from businesses like yours which have been able to increase their bottom line by decreasing their tax burden. You ought to be able to go to the Better Business Bureau and not see complaints about the way your vendor conducts its business. Remember—this is your taxes. Only being mean to your customers and spending money while generating no income will put you out of business faster than screwing up your taxes. Got it?
Track record is important. A tax credit services provider that isn't up-to-date with current credits, or doesn't know how to apply them, will not save your business any money. Equally, a service that has a history of applying all of the credits available, and letting the IRS check whether they should be applied is more likely to land you with an audit, and potentially fines. The thin line you have to walk if you were doing this yourself is the same line that your provider will need to walk.
If you can't get your vendor on the phone to ask questions, if they aren't being proactive about informing you of new credits and how they will impact your business, if they don't understand your business ... these are signs that the question you should be asking is “Do I have the right vendor for my business?”