Boot off the CD/DVD. At the "boot:" prompt, enter
"memtest86" and off
you go.
Is there a "memtestppc", as well?
Listing bootable images showed a "check". When I gave that a run, it
went for a while, to something more than 70% and then wandered off
into the ether -- black screen, as if it openfirmware had put it to
sleep, but it wouldn't wake up.
The Fedora 9 powerPC netinstall doesn't seem to have any alternative
images at all.