This may help http://www.fs.usda.gov/detailfull/gi...777&width=full
Also generally speaking (this may not apply in the Gifrod Pinchot NF) if the road is bermed with snow it is closed to wheeled vehicles. If the road has been groomed for either snowmobiles, cross country skiers, or snow shoe users it is closed to wheeled vehicles. Normally such areas are marked by signage but that does not always apply. It comes down to contacting the Ranger District office and confirming where you may go.
By all means either download the MVUM or get the one from the Ranger District office (free). It shows only the roads and trails that are open. Everything else (not shown on the map) is considered closed.