I start with google earth, mangani's bigfoot overlay and a topographic overlay. I add in USFS wilderness maps for trails to-from specific locations. Then I search the old archives online for older versions of maps that have now-abandoned trails. These are usually USFS or USGS. If I need to worry about property boundaries, my county has tax lot mapping online.
The maps I've found give a general idea but are not accurate enough or detailed enough ... gotta put boots on the ground. Three older trails I've been looking for are at least a quarter mile away from where they appear on the maps. In heavy forest that's out of sight from where they're "supposed" to be so .. not good enough. Of them, I've finally traced through one end to end, another I've found all but about a 200 yard section (guessing there's a switchback in a meadow that gets a lot of snow which has destroyed the trail), and one .. I've barely found anything, not a trail, just a sense of where one reasonably would have been. Two others I expect to be similar. One of those leads near a series of long, thin lakes, partly seasonal. I think they lie in a fault line because they seem to go on for miles with no deviation. I suspect that's going to be a "spot" at certain times of year. Water, cover, no humans. One is on his way soon though.