Team MightyBoy

When I pulled the gauges from my Evo to sell I held back from selling the oil temp gauge as it wasn't really working. But I've had a look at it and it looks like it's just a wire that went into the harness going into the gauge that was pulled out, and I may be able to get it back in there.

But do I have to solder in the wire to the ignition in order to test if it's works now, or is there a quick way (I dunno, maybe using a battery or some complex system of mirrors and pulleys!) to check if it's functioning?

as long as you can hold the wire in place you will find if it works - no need for solder, crimp lugs or anything else until you need a permanent connection. To my knowledge an oil temp switch will change resistance in accordance with the temperature of the oil.
I would be careful about applying a full 12V to the guage without knowing that it will take it. I have blown the voltage regulator in a dash pod by doing so.- talk to an auto electrician first.