This is one time that "close enough is good enough". Square your ruler on the center fold of full width fabric. You can probably look down the length of the ruler and see that you are pretty close to lined up with the fabric threads. If so, you are on straight of grain. Same thing applies working with the length of the fabric, except there is no center fold.
Straight of grain will be easier to cut, sew, and iron as it won't stretch totally out of shape like bias can.
Sometimes you will want to use stripes, plaids, or checks. Then don't worry about straight of grain, just line up your ruler with the print.
Most of us are saying the same thing, but different people understand differently. I hope one of these makes sense to you.