measuring balanced output

In a piece of commercial electronics I have an amplifier cassette for a broadcast mixer. The input is unbalanced, but the output is a balanced line. THe amplifier actually contains 4 sections, 2 stereo amps. which I need to adjust for the same amplification.
How do I measure the balanced output?
The input is easy, my signal generator can be switched to an unbalanced output, but what about the balanced output?
Do I connect my RMS voltmeter across the two balanced leads (which will probably ground one of them thru the unbalanced nature of the voltmeter)??
Do I measure against ground?

Uwe

If your voltmeter is unbalanced, one way is to use a 600Ω to 600Ω audio transformer. Connect the primary between the two unbalanced outputs, and the secondary to the voltmeter/o'scope.

My o'scope can do the A-B differential reading trick.

Yet another way is to build an opamp differential amplifier.