How to see if there's DC at my AC outlets?

1ST Setting your voltmeter to DC and plugging it into and AC outlet will probably smoke the unit.
2ND Direct Current is not ever present in Alternating Current line nor visa versa
3RD Try isolating the amp or another outlet. Have you tried a cheater plug elimnating the ground?

"1ST Setting your voltmeter to DC and plugging it into and AC outlet will probably smoke the unit."

There will be no smoke, that will be fine if you want to try it.

"2ND Direct Current is not ever present in Alternating Current line nor visa versa."

Sorry, that's not true.

The buzz may be caused by noise on the line. A common culprit is a dimmer switch. I used to have an isolation transformer that buzzed loudly when the light in my stove hood was dimmed.

What if I test the hot to ground or neutral to ground?

I tried isolating just the amp, with nothing else plugged in, and the variac still hums / buzzes.

PS Audio has a product called the Humbuster that stops DC from getting to your electronics which causes the transformers to hum. So how does the DC get into the lines, then?

Did you try going around your house and turning everything off to see if it stops? If you are in an apartment it could be your neighbors stuff.

Does the variac hum with nothing plugged into it? If it only hums with something plugged into it then something might be loose in the Variac, but I still think noise on the line is the most likely culprit.

What's with the Variac? In my experience, all Variacs buzz, but some buzz more than others.

El, I have a Variac that has never buzzed that I noticed, but it is rated at 15 amps and I probably never got close to that limit.

Mark, now that I think about it, why are you using a Variac?

It comes as part of the Joule VZN-100s amps. There are the two monoblocks, and they are connected to Joule's variac. It supplies controlled AC voltage to the monoblocks. I suppose this way your amps always get the proper voltage that you dial in (64 to 65 volts AC), and you're not constrained by the higher and lower voltage supplied by the power company.

The outlets in my Clear Image T4 filter have 2 MOhm resistors at the the hot & neutral of the outlets. A friend told me they're for filtering DC out...

PS Audio makes a "humbuster". It appears to be a DC blocking device and is reasonably priced (ay least as far as can be said for an audiophile component). PS Audio claims it will quiet mechanical transformer noise. They are designed for high power loads so a power amp should be OK. Maybe a dealer would let you borrow one to see if that does it.