The first way to tell, is if you have to wait the 20 or so seconds when you first turn the key, rather than just 2 or 3 when the light goes off, your block heater is not working.
The second way, is if it is quiet, when you plug in the cord, you should hear the water start sizzling inside the block.
the third way (which I learned by accident), is get the plug wet, stand outside barefoot, in a puddle such as in rain, plug it in, and see if you feel a little tingle in your hand. If you do, It works! I am sure someone else will chime in with other ideas and tips, but you should not have to wait for the manifold heaters to cycle on when you first start up the motor. Once you start it, of course they will cycle for a few minutes, or until you go over 20 mph. Good luck!
-Chris-