If yours is like mine, one of the lines can be traced to the vicinity of water pump. That should be the supply. A trick I used when plumbing my heater back in was to buy a length of clear vinyl tubing with the same ID from the local hardware store and to plumb it in where the heater core would be with a purge faucet.
It allowed me to see the progress of the bleeding and purging.
Something like that might help you as well.