Step one:
Look for a shop that has a Serotta fit bike. It's not the quintessential tool but it does help the phone calling process to weed out the bull from the real deal.

Step two:
Ask if anyone there is Serotta fit certified.

Step three:
Ask what different types of fits they do.

Fit type one is the least expensive and the most bang for the buck. It should entail a seat height, knee over pedal spindle (KOPS), reach from the saddle to the bars (horizontal), drop from the saddle to the bars (vertical).

The first two items should have very little variance. IOW, it's a set measurement that may be adjusted by only millimeters.

The last two items are more about comfort and purpose of riding. So, there's more variance in these parameters.

There should also be a discussion about what length of stem will work with that bike before the bike's handling becomes adversely affected.

Fit type two is a pedal cleat fit. I believe this is often over-looked. Just search here on foot problems that people have while cycling. Tons of foot problems.

Fit type three is a comprehensive fit that includes type one and type two and then goes even further to the point of dialing in the tubing lengths for a custom frame. This is usually included in the price of a custom frame order.

The X fit is a rare fit. Guys like Andy Pruitt will take their Chiro expertise and medical knowledge to dial in a fit that usually only the very best pros have access to. But Andy will fit you too if you want. I believe his fit comes with a pair of orthotics.