I use the DEC's "Mountain Bike Trails" Google Earth layer available here:
http://www.dec.ny.gov/pubs/103457.htmlAlso, "Mountain Bikes are Permitted" is not the same as "Mountain Bikes are a good method of travel" for most of these routes. Some have been beautiful backwoods trails, some are forest roads with little character or scenery, and some turn into a Hike-A-Bike Slog. You never can really tell. The best mountain biking remains on purpose-built mountain bike trails by BETA or SMBA or Wilderness Property Management or whatever group works on local trails.
If you're looking for an adventure, though, some of DEC trails will definitely give you that. Just be safe and budget extra time when planning your trips.