The viscous clutch test I believe involves jacking up a wheel and trying to turn it with a long bar to verify it is free.
I have on two, what I did was (following a bit of research): jack up one front wheel, handbrake off and in neutral, slowly try to turn wheel. With a lot of effort I got my GEMS to slightly turn by hand, bar on wheel nut or centre - constant slow and steady force - turned wheel around half a turn. It takes a while. There was some rebound on an inch or so and if you speed up or sharply push/pull it all locks a bit. I also have a worn drivetrain with a bit of slack in it. (On the list of jobs). Also - check for wind-up as it lifts off the ground, shouldn't unwind as the wheel comes up.
I have seen various calculations of torque wrench settings and bar lengths for wheel nuts vs centre nut, timings and so on, but not sat and worked it all out. The Thor, which is higher mileage was marginally easier to turn and seems to have less slack in it. Both will turn easily with a longer (3' or so) bar, but that's apparently not a good test. (I am not heavily built, was trying things out). Someone will know the exact figures, I think mine (esp. the GEMS car) take just a little too much force, but that's possibly wrong. 1'6" bar and gentle steady heave seems to do it.
Car park test (tight lock, dry ground, slight accelerator etc) seems to be a good bet as well, although it'd be useful to have a benchmark of how it ought to feel - as in see what it's like with a new VC and compare. My GEMS has a dodgy transfer case seemingly, so I am anticipating taking it apart at some point anyway - fluid drains with a silver sheen which isn't promising. It's been a workhorse/towing car for most of its life, and not looked after. I have just cut out and welded up a rust hole in the boot floor beside the wheel well. Looks like the back end's been stuck out of a beach hut for years tbh. The Thor and GEMS both did similar things on the viscous test I did, although I'm not 100% convinced either is quite right.