Formal tours are offered at Saratoga Springs only. VERY low pressure. If you schedule it in advance, Disney will even send a car to pick you up anywhere at WDW (your resort, theme park, etc.) and they will drive you back (or elsewhere) when the tour is complete.
Even if you don't think you are interested in Saratoga Springs, it's worth taking the tour. They will explain the program to you and answer any questions you may have. If you want the name of a great
DVC Guide (salesman), feel free to send me a PM.
For the other resorts, you are welcome to visit on your own. The Beach Club and Boardwalk are just outside of Epcot. Wilderness Lodge can be reached by boat from the Magic Kingdom. Old Key West is best reached by bus from one of the theme parks or Downtown Disney.
You will be welcome to roam the resort grounds. If you want to tour a room, you'll have to ask at the front desk. If they aren't busy and a room is unoccupied, most times they will be happy to give you a look.
In terms of floor plan, BCV, BWV and VWL rooms are virtually identical to the models at Saratoga Springs. But the furnishings do vary greatly. Old Key West was the first DVC property and its rooms are MUCH larger--particularly noticable in the 1B and 2B units.
Hope that helps.