You can measure your sit bones in a number of different ways: 1.if you have thick carpeting on your steps sit on one then measure middle to middle of the divots you leave
2. sit on a step with tin foil stretched across and again measure.
3. have also heard you can do the same by putting down baby powder if you have hardwood steps.
4. lay on your back, bend both legs and pull them into your chest, have your S.O id the sit bones and measure.

If you research your issue on the internet you will find that sit bone width is not necessarily the be all end all according to many fitters. It may help and it may not. In my case I have had problems with both wide seats that are supposed to fit me and more narrow seats. I am still searching for just the right saddle after going through a half dozen and getting a pro fit done. Good luck with your search!