Lucky,
Everything attached to the input to the pump is called the "Suction Side" The pipes going back to the pool are called the "Return Side"...
I am assuming there are only two pipes coming out the ground and going into your pump... (Hard to tell looking down from the top..)
If you know for sure that the pipe on the right of the Jandy valve is from your skimmer, then that means that the pipe on the left side of the Jandy valve has to be from either the main drain or a vacuum port.. or a 2nd skimmer if you have one of those.
A vacuum port would be about a foot deep on the side wall of the pool and should have a spring loaded cover, which could be missing.. If you do not know if you have a vacuum line, you can just go to each pipe coming into your pool and make sure you can feel water coming back into the pool... If you find one that has a cap or has water flow out of the pool that would most likely be a vacuum port.
You can also just shut off the Jandy valve and see what happens... Does the pump still have plenty of water? Does the skimmer still work, can you see water flow?
If the Jandy valve controls your Main Drain, then it should not make any difference if you shut it off.. If it is for the main drain, I suggest that you only open it about 10%.. This will allow your skimmer to work better.
Thanks,
Jim R.