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

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?

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.