^^^ Assuming the speaker looks "nice" at that frequency... Ideally*, you'd use an RMS voltmeter with enough bandwidth to measure a number of points across the audio band. In practice, a spot check at 60 Hz is not unreasonable, although sensitivity is typically measured at 1 kHz. If your speaker's bandwidth does not extend to 60 Hz you may get a misleading result. Of course, you'll also get a misleading result if your DMM does not have 1 kHz AC bandwidth...
Either way, do it under load, as Ricci said. - Don
* Actually, "ideally" you'd use a wideband vector meter system to determine V, I, and power, but those are not cheap.
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley