There's a couple of tests mentioned on this page:
http://www.cleanmpg.com/forums/showthread.php?t=17560
They involve checking the resistance accross the plug prongs, and a real simple one: after plug-in (it has to be real quiet, no ambient noise, say in a garage) and within about 30 second, you should hear this sort-of plonk-hiss sound, kind of like a kettle waking up.
Yes, after two hours, if I pop the hood and feel, everything feels kind of warm, in particular the top of the radiator.
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