Question

You want to use a mulimeter to test a power suppl'ys voltage. to do this, you need to turn on the power supply.

Answer 1

Yes.  That's a fact.  Truer words were never spoken.  If you expect
to ever measure the output voltage of the power supply, then you're
eventually going to have to turn it on.  There is no other choice.

You'll also need to connect the test leads of the mulimeter to the
output terminals of the power supply at some point in the process,
AND turn on the mulimeter.