There is declination marker at the bottom of topo maps. The problem with that is it is very small and when taking a reading in bad weather you can easily be off by a few degrees. Over 500' it wouldn't matter much but over a mile it could mean life or death. It is always much easier to spread a map out in the warmth and comfort of home and draw the magnetic lines.
You can also do it on the computer with mapping software and and then zero im on the particular area you are going. That way, the parallel lines are printed on the map itself.