Thank you for all of the suggestions.

I tried the at-home pressure test.  Engine was cold.   I removed the
expansion cap to make sure I wasn't starting with pressure in the
system.  Replaced the expansion cap and started the engine.  I let it
run for about 1 minute.  I revved it a little from the engine bay,
trying to see some of the bubbles in the expansion tank that I've seen
before.  I shut it down and let it sit for 30 seconds.  When I removed
the expansion cap, I heard a small rush of air and I noticed the
coolant level in the expansion tank rise up a little bit.

So, it could be the pressure was caused by heated air trapped in the
system, or it could be exhaust gas being forced into the system.  I'll
call around to see if any shops nearby can sniff inside the expansion
tank.

If the sniff test is positive, what is the best way to isolate which
cylinder head gasket may be leaking?

Thanks,
Travis