How do I know if the block heater is working?

Take it out for a drink. Whisper in its ear. If it get's hot and bothered, it's working.

 

Oh Geez, I don't know maybe the block will be warm to the touch.??

 

Plug it in with the engine cold and leave it for 10 minutes or so. Then feel the engine block around the block heater. It should be noticeably warmer than the rest of the engine block.

 

if it is -20 and starts in the morning it is ok

 

After plugging it in I always stick my head under passenger side fender and can hear a slight hissing sound.

 

I've got a scan guage, that reads lower than the dash guage.
After plugging it in, on a dead cold day at 10 degrees, I came back a few hours later to find it reading 75 on EOT.

That, and what @NorthernMechanic said.

 

When you plug it in, slowly put the ends together and you should hear a spark noise when the cord connect. Unplug it and should hear it again when the cords separate.

 

Turn Ignition On, & read Temp Gauge .........

 

10 degrees is dead cold? You need to come up to my neck of the woods.