Plug in the power adapter ensuring that the two leads do not touch. Attach the multimeter to leads. Basically if it shows the correct voltage as a positive value, the orientation of + and - is correct. If the multimeter reads a negative value, you have the polarity incorrect.

Once you have figured out the correct + and -, make a note of which is which. You can put a dab of red nailpolish on the + wire.

Remember that the voltage is 29.4VDC. You have to set your multimeter correctly. See that I set mine to the 200V range - because the 20V range would be too low.