The sender will have a resistance value when cold and another when up to temperature.
These two extremes should be what the temperature gauge is expecting. For example (these will not be correct, so you will need to measure these yourself)
Cold - sender reading is 1000 ohms
Hot - Sender now says 200 ohms (Could be other way round, but you get the idea)
Now you can buy a potentiometer from maplins for the larger value and pretend that is the sender and connect this to the gauge and when you turn the knob, the gauge should vary from cold -> hot etc.
(You can also buy a set of resistors, but you need to know the colour code to read these, Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White 0,1,2,3,4,5,6,7,8,9)
The gauge needs to match the sender ideally, but with some electronic knowledge, they can be made to match
The pot above shouldn't cost more than £1 (but, knowing maplin - £2 or so. The cheap LINEAR one is fine - don't get the LOG one if you see it)
If you can measure the resistances with the multimeter when hot/cold and the gauge works with the variable resistor, you can see if they match.
Simon.