That's a great question. Here's a simple way to tell the difference:
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Normal Condensation: This is moisture that forms on the inside room surface of the glass. It's caused by high indoor humidity and can be wiped away. It means your windows are doing a good job of keeping cold air out.
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Sign of a Busted Window (Seal Failure): This is fog or condensation trapped between the two panes of glass. You cannot wipe it away. This means the insulating seal has broken, allowing moisture inside the airspace. The window has lost its energy efficiency.
What you're describing—fog between the panes—is a clear sign of seal failure. It's not due to rain or humidity in your house; it's a manufacturing defect in the sealed glass unit itself. The solution is to have the insulated glass unit (IGU) replaced.