Well.......the easiest way is to go to your local bike shop and ask them if they have a butt-o-meter. It's a cool device that you sit on and it measures your sit bones. Then you can determine what saddle is the right width for you.

A more difficult way is to take a piece of typing paper, place it on a stool or chair and see if you can make indentions with your sit bones. One TE'er had luck using Play Do to make her sit bone impressions.

Good luck and let us know what you find out.