An AVO Model 8 on the 1000volt ac range connected between Active & Earth is a good tester for RCDs.
Many moons ago,the Sparkies came in & installed an RCD on the workbench while I wasn't around.
I had been working on a bit of faulty tube equipment.
My usual habit was to check the incoming mains by hanging the AVO between Active & Earth,& Neutral & Earth in turn,rather than test across Active & Neutral,as it was more convenient to just poke around with one probe,with the other lead securely clipped to Earth.
Imagine my surprise,when the whole bench turned off!
There I was,scratching my head trying to figure out what could be wrong with the AVO to make what I thought was a new Circuit Breaker drop out.
All was revealed when I took a closer look at the "Circuit Breaker"!
That was my introduction to the wonderful world of RCDs!
VK6ZGO