First, if you're going on fuse box labels, it sounds as though you might want to d/l an owner's manual:
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You can tell positively if the compressor is not running by observing it. There's what appears to be a disk in the center of the spinning outer bit, the portion that's driven by the belt and turns all the time. When the compressor is off, the disk is stationary. When it kicks in, that disk should click and start turning.
If it's like the Denso system on my Avalon, the Defrost setting is a second circuit to the a/c compressor. Check to see if the compressor kicks in when somebody selects Defrost on the dash. If so, your a/c control might be unhappy.
I'm pretty sure the next step for diagnosis is to check the system pressure. You can do that yourself, but it requires a gauge manifold (Harbor Freight used to have a fairly inexpensive set). There are lots of youTube videos on how to diagnose with an a/c manifold, and what the low and high pressures should be.