I'd guess that the 220V input goes more or less straight into a transformer. With a multimeter set for resistance measurement, this would show as a permanently low resistance between the terminals (in the order of a few ohms max).
You might need to measure for a while (10-30 seconds) though, because the 13.8V input *may* have a large-ish capacitor behind it. This will cause a relatively large current to flow initially when measuring, resulting in a low resistance measurement. When the capacitor fills up, that current will diminish and the measured resistance will go up. It's not certain though that such a buffer capacitor is present, because the car battery never really goes below a certain voltage once the car is running.
If there's only one pair of terminals that has a constant, low resistance between them, those must be the 220V terminals.
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