I don't have a clamp on meter, so I can't try that--even if I did, the cord on the heater is moulded and would be very difficult to split.
I know you said you didn't have a clamp-on current meter but in case anyone else does and struggles with splitting leads you might want to try one of these gizmos. There are many different brands but they all do the same thing - allow you to test AC loads without splitting wires or tearing the outlet box apart. I have one and it works great!
