how do you know when yoghurt is fully dehydrated

I have just dehydtrated a kilo of high fat greek yoghurt. I am not sure if it is ready or not. A beef casserole and some fruit are ready, that went into the dehydrator with the yoghurt. The yoghurt has reduced to a about a 5th in weight and now weighs 200 gm. Any surefire methods for testing dehydrated yoghurt?