An aggressive preparer will get you every dime of deduction, but may open you up to audits. An audit is not a problem, providing they are willing to back up their work and defend it without additional costs to you. Find out what their experience has been with audits and their success rate. That will give you an idea of their expertise level and how much you can trust their assertions on your tax return. Whether or not they are certified can provide you with information on how well they know their information.
Most certifying exams are at least two days long and cover a lot of information.
The vast majority of individuals becoming certified have to take the exams more than once. The fewer times the person takes the exam, the better they learned the information. Any person who holds themselves out to be a professional should have at least 5 years in the profession working with the areas they are going to be providing to you. See that they have experience in your type of business or similar businesses of your size. No one is an expert at everything.