Follow the lines for which way flow is going. Should be easy to find the rear brake line, and crack it loose at the p-valve, and see if anything comes out. Step gently on the pedal, or all it will do is offset the safety valve in the p-valve.
If you don't get any fluid out there, crack open the FEED line for the rear circuit, (which I think is actually the FRONT line on the M/C. See if you get fluid flow there.
Just crack the lines loose a bit, and have someone step down on the pedal, and HOLD it there. Re-tighten the line before they let the pedal up, to keep from sucking air into the system.