Depending on where you live, the people who build and maintain the roads may have already done the surveying for you and marked them with mile markers. I like to do a roll-down test: I check my odometer reading at the mile marker near the top of the ridge and roll down two miles. If correction is needed, I can get within 0.5% this way.
Note this depends on where you live and ride. In some parts of the midwest, they seem to put up mile markers like some people put up bouquets of balloons to celebrate a child's birthday. (Maybe that explains some of the numbering there, too!)