The trails can be LOST/CLOSED. The land owner will not allow use of the land for the trail anymore. Lets see, a hunter can get pissed and shoot at you. I have heard of stories of hunters getting pissed and firing a few shots in a sledders direction. The no fun cops can give you nice large tickets.
Here is a way that you can find out
http://www.travelwisconsin.com/snow_report.aspx
They can be a few hours off. But the best bet is to find the numbers to local clubs and call them up.
Here is a club in Grant County The first page tells you if the trails are open or not
http://rrhuehn.fatcow.com/index.html
or call this number for trail conditions (608) 723-2125
Dane county (Madison) Phone for maps and trail conditions
After hours: (608) 242-4576
Days: (608) 246-3896
Rock County (Janesville) Snowmobile Hotline message 24/7
(608) 757-5458
Final note, go to this link
http://www.awsc.org/Dynamic_Menu/Snowmobile_Clubs.aspx click on the county you want to ride. If you look on the right hand side you will see a number to call for the county you clicked.