the way how I remember it is as follows:
at one end, you'll find some thicker black, red and yellow sets of wires. these are the power ones, A|1, B|2, C|3 etc. you can also tell by the notch in the JAMMA edge, the first 6 columns of pins are for said power wiring (2 grounds in a column, 2 +5's in a column, 1 column of -5, and the last column is for the yellow +12V wires) ... you'll then observe that the JAMMA harness has a plastic "key" in the H|7 column. from there just count over 8 columns, and that'll be your 15 Test wire.
I admit I cheated and read this:
http://www.jammaboards.com/jcenter_jamma_pinout.html
but that's generally how I remember which orientation the JAMMA harness goes on, the power wiring goes on the end where the notch is. it's also worth noting, if you haven't figured this out yet, that if you're facing the JAMMA edge from behind with the power wiring on the left hand side, the top row of pins is the "Parts" side, and the bottom row is the "Solder" side.
in this picture, this would be the "Parts" side
just pretend that the whole thing is rotated 180 degrees.
on that particular harness, it's the "orange" wire just past the video signal wiring.