1. Find flat, hard surface (pavement..)
2. Sit & Wait

Get to know what the sit bones feel like on a hard surface - then get back on your saddle. Can you find your sitbones on the saddle now? Roll your pelvis back - see if they make contact w/ the saddle. A few of us have sit bones that are far wider than the majority of saddles on the market will support ~ when I rode a saddle that didn't fit (search Brooks for more detail) I had bruising pain & numbness in that region. Yowza.

If there's a shop in your area that carries Specialized, they may have a gel tool used to help measure your sit bone distance.. in case you want a more scientific approach.

I'll also second a professional bike fitting.