> OK, so I have access to an oscilliscope on loan.  :)

Answer 1. oscilloscope
                ^
> I need to watch the 120 VAC line, and also monitor the other side of an
> optoisolator, which is, of course, supplied from a +5 VDC circuit that
> shares no common ground with the AC side.

Answer 2.

Use a small transformer from 120 VAC to 6 or 12 volts. Monitor the output
of that instead of the 120VAC line.

Do Not connect your 'scope to the power line. Do Not float your 'scope by
lifing the ground prong.

Cheerful regards,

Bob


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