You can do this with a $4 Centech DVM from Harbor Freight. No need to drag in an oscilloscope.
What I do is design all my stomp box power supplies with +9 and -9 and install a pin header (commonly used on computer boards and old hard drives) with 3 rows of pins to choose the polarity of each output jack. You use a little jumper that comes on the old hard drives to select which polarity goes to each output.
This is probably a useless idea for the needs of this Thread Starter.
