Put your hand on the thermostat housing on a cold morning . It should be warm.
Even when plugged in on a cold mornin 6.5s are noisy. But the heater shound start blowing warm air soon after start up if the block heater is working.
Just let it idle a couple of minutes till the piston slap goes away and motor on.

 

I just listen for the coolant to spot boil when the heater first is plugged in.
The coolant will sound like it is sizzling when you first power it up because it can transfer the heat fast enough.

 

Unplug the extension cord & measure resistance through the block heater. It should be around 25 Ohms.

 

Usually just the simple sparking when you first plug in will tell you you're hot.Listening to the crackling will also confirm.

 

One morning I unplugged it and the block heater cord was warm and the truck started fine, the next morning when I unplugged it the block heater cord was cold and the truck started like it was cold.

 

Check the outlet, any extension cord you may be using and the heater cord. If all are ok, I think your last post answered your question. Just like a home water heater, the block heater is good for only so many cycles.