You can measure the full width of your roll of fabric, but that is not actually the measurement that is most useful in development and production. What you really want to know is the cuttable width of the fabric.
The cuttable width is the usable width of the roll of fabric between the selvedge edges. This width is usually about two inches narrower than the full width of the fabric. Each fabric and roll is slightly different, though, so it is best to measure each time you order a new lot.
When you are looking at a fabric header, note whether the width is listed as cuttable or not. If it isn’t, plan to subtract those few inches from the width to estimate the cuttable width of the material.