When a viscous fan is cool the fluid doesn't 'drive' the fan, such that when you first turn an engine on, it should be easy to stop the fan with, say, a rolled up newspaper (or something that won't hurt you or it!). As the motor warms up and the viscous fluid also warms up the fluids viscosity changes and it starts to drive the fan, with some 'slippage' of the fan, but it would be much harder to stop. As it gets even warmer at some point it will completely lock and drive the fan directly and will be impossible to stop without damage.
You can test this by first thing in the morning the viscous fan should be relatively easy to turn by hand. There might be a very little drag, but notalot. If it's very stiff to turn and tries to turn the belt/pulleys then it's probably nackered.
Go for a drive and get the engine properly hot, so not just for 5 minutes or so, but at least half an hour, preferably in the sun and stop start, or motorway thrash. Stop the engine and try to turn the fan now and it should probably turn the belt first! If the fan turns the same as it does when cold it's probably nackered.
