If it is paper, find a transparent piece, such as plastic, with several parallel lines on it.

When you look at it, find two adjacent contour lines and match the two parallel lines with the contour lines or points.

Look at the position of the point to obtain data, and you can calculate or estimate it. You may understand it by doing it, and just look at a few known points.

If there is a file, then with cass, you can directly point out the elevation data of this position. These are also estimates, because by default, the contours on both sides are straight.