If you have tickmarks on the segment, on a blank piece of paper, then you count out the spaces to get the length of the segment. This is assuming the tickmarks are properly spaced out. If there aren't any tickmarks, then you'll have to use a ruler to find the length. Either way, a ruler is encouraged.
The coordinate plane makes things easier to find the length of any segment. Use either the pythagorean theorem or the distance formula to find the length of the segment.
