+1 to Jim
My sit-bones are about 150mm apart, so narrow saddles don't actually support the hard bits. My LBS had a Specialized sit-bone measurement device that was relatively clever. Made of some kind of gel that you sit in and it captures the impression of your pelvic bones. Measure c-c and you get some sense of how wide a seat has to be.

Kind of weird, but I found it accurate when I tested out narrower seats than my bones. Obviously, this won't help you assess the comfort of the actual surface or cushioning, but it does help.