On my first ride of the Mighty Scura, I was at 165 miles, and thought.. Hmmm, I wonder if that low fuel light is working?" It wasn't. Fortunately, I found a gas station shortly.
To test the light, just ground the plug at the switch. Those thermisters are apparently pretty fragile. Mark had a box of switches at MG Classics and told me to see if I could find a good one. I ohmed them all out, and found one with some resistance. It's still in it, but I had to put a different connector on it.