The switch on the thottle simply goes from 'open' to 'closed' i.e. 0 ohms to infinity ohms.
Another way to test it and its connection (thus eliminating more options) is this, it only works on an auto:
Drive at 30mph in '3', and shift from '3' to '2' with your foot off the accelerator. If it bangs down into 2nd gear with a jolt, your switch is working OK for zero throttle. If you do the same thing with even a tiny bit of throttle, it should slip quite a bit, letting you rev the engine fairly freely, and not instantly engage 2nd gear with a thump.
If you can get these 2 different behaviours, the switch is fine.
Why the switch causes this different transmission behaviour I don't know, as its primary function is to shut off fuel to the engine when you are coasting.
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black