I thought I'd actually answer your question...
To test the sender, (or the gauge) first find out from VDO what the resistance range of the gauge and sender are. typeically you'll find numbers between 50 and 200 ohms.
Once you know that, to test the sender and all of it's wiring, simply disconnect the sender wire from your gauge and hook an ohm meter from that wire to ground.
Measure the resistance with the engine stopped, and with the engine running- the value should change radically between those two situations, and somewhat match up with the resistances you got from the manufacturer. If the resistance is always zero (or very few ohms) your wire is probably shorted... if it's infinite or higher than a few hundred ohms it's probably broken or intermittent, and if it doesnt change between running and stopped (it should follow your engine speed just like the gauge will when it's working) then your sender is bad. You can disconnect the sender and do the same test right at the sender to verify if your wiring is kaput...
To test the gauge, get a potentiometer with roughly the resistance that the manufacturer told you, hook it between ground and the gauge input, and vary it. The gauge should follow along... again to test the wiring and the gauge, you can put the potentiometer back where the sender is (disconnect the sender) and it should make the gauge wiggle happily from back there...
If you can't figure out the resistance value from VDO, it's probably good enough to start with a 500 ohm potentiometer- that may not match up entirely well but it will give you a good idea if things are working or not, and which ones aren't
If this is clear as mud, let me know and I'll try to reword it... to me it's clear but this stuff is what I do... explaining it is another story!!!