A plugged in block heater should start to feel warm within 1-2 minutes if you put your bare hand on it. At the most it should take 3-4 minutes. That’s the simplest and best way to check them. If your hearing is good, you should be able to hear them start to make sound in about 15-20 seconds. When I was farming, we kept a spare block heater on hand for our feed tractor and our loaders, because at -35 degrees Fahrenheit, especially in back in the 70’s and 80’s, they didn’t start without plugging them in.