Classic USGS topographic maps can include a very large number of symbols so it is not practical to print a legend with all of them directly on the custom maps. 

The full USGS legend is available at:
https://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf

We suggest printing one of these for reference when examining a classic USGS topographic map.

Note that newer US Topo maps do include a key to symbols used on that particular quad.