Ir guns will work fine on the bed, but not pin point targets. The simple explanation is that they measure a cone of temperatures out from the sensor. Think shotgun not rifle. So aiming it at the heated bed, you'll get the average of the entire heated bed, which is what you want. If you try to aim it at the extruder, then unless you are almost right up against it, you get readings from the air and framework around it.

If you want a simple way to test the hot wnd's temp, get a multi meter that can do temps (everybody should have a multi meter in their toolkit), and attach the thermocouple.