Feel...
How it steers, how it brakes, how it accelerates off the saddle. Behavior issues really and "fit" comes into that. A bike that wheelies nice usually has good air-born qualities, if that matters.
Test a bike that has the majority of parts or specs you already want (or need). Less upgrading, the better.
Disc brakes usually get stronger AFTER some breaking-in period. Most LBS display bikes have disc brakes that aren't broken in yet, so make some allowance for that.
Then I ask myself:
• does it please me every time I look at it, or is there something about it that bothers me?
• will it be relevant 5 years from now?
• will it make my wife happy?
• does it smell good?
(I kid)
.