i have a 2013 chevy 1 ton dually, doesel of course. after plugging in my block heater over night, it seemed hard to start. up untill now everything seemen fine. i do have 120 volts at my plug in.
One way to do a simple test without tools is to unplug it quickly in a dark setting and see if there is a spark. That will show if it is drawing power.
Also, are you saying that it started harder when using the block heater than it did in the past without using it? Sometimes the block heater puts enough warmth in the engine that it won't call for the glow plugs to cycle.