with mine the first time, it was easy to tell. they were brand new rubicon express leafs, but were physically different from the left pack to the right. one was rounded ends, the other was squared off ends. so they replaced both with a matching set.
but then the following season, passenger side one went a little flat, putting the front drivers corner up high. after sending them pics, they once again, sent me out a matching set. rubicon express is top notch in customer service in my experience, so that's why i will always look to them first for any suspension components.

you should be able to tell by eye, which one is flat or sagging. but measure each side from axle tube to frame rail, or pull both and lay them flat beside each other. i pretty much guarantee it's one of your leaf packs.