Many bike shops have what we like to call 'bum-ometers' (we just made that up). These are devices that you can sit on and feature a memory foam. Your sit bones leave an impression in foam, the width of which can be measured.
A general rule of thumb is that the saddle width should be sit bone width +2cm. Specialized and Bontrager both produce devices like this to measure your sit bones and these are often available to try in your local bike shop.
Alternatively, if you don't have access to a bum-ometer you could always improvise with some Playdoh and a sheet of paper or a piece of cardboard. It's an important thing to remember that a bigger derriere doesn't necessarily mean you have bigger/wider sit bones.