I am not aware of how to use a DVOM for testing speaker polarity, but I do have a solution:
1. Ensure that the speaker lead points you have access to go directly to the speaker only. That is, be sure that the leads go only to speaker, and that they do not tie into any other wire harness, power point, etcetera.
2. Get a 1.5 VDC battery (i.e, a "C" size).
3. For a brief moment, touch the "+" side of the battery to one speaker lead, and touch the "-" side of the battery to the other speaker lead.
4. If the speaker cone moves out (i.e., away from the speaker magnet on the back of the speaker), then the speaker lead going to the "+" side of the battery is the "+" side of the speaker, and the speaker lead going to the "-" side of the battery is the "-" side of the speaker. If the cone moves in (i.e., towards the speaker magnet on the back of the speaker), then the speaker lead going to the "+" side of the battery is the "-" side of the speaker, and the speaker lead going to the "-" side of the battery is the "+" side of the speaker.