Except for private property, you are supposed to have a valid registration. For older bikes like your '96 RM250 and some post '02 bikes, you get a green sticker. Most post '02 bikes such as your RM85 have red stickers and are restricted from many areas in summer, supposedly due to air quality reasons.
If you are from out of state and your bike is registered in another state, there are provisions for you to ride.
Attached is the Red Sticker schedule.
Most of the places shown on the schedule either have rangers or entry gates that are staffed. If you come across a ranger, they will check for a current registration on any bike they see.
So you either pay to ride on private property, or you pay for your registration.