The trails listed on USGS maps differ somewhat between the early (1880s) maps, 15 min maps and 71/2 maps. The first two series were never completed and the last are rarely field checked hence things can be missing such as trails, buildings and things not seen on aerial photos. I found a mining map one time that had all the pack trails in the 1800s. Very few were on the more modern maps. I've found pack trails that have existed since the 1930s and don't show up on any modern maps.
Only time on the ground gives you a chance in heavily timbered areas and if they have been logged, they are likely gone forever.