How do you measure AC voltage in a circuit?

[The Dude]

If a meter has a frequency bandwidth of 1kHz, readings may be low by 30% (= -3dB) @1kHz.
So it would be better to use a 400Hz test signal.

I like to do that anyway. To my ears, 400Hz is more pleasant to listen to than 1kHz. Of course, if you are measuring for extended periods of time, a dummy load solves that problem. I'll also add that, generally speaking (though not always), you are looking for a LARGE discrepancy in measurements vs schematic, so measurements are not super critical.