How do I know which of the Air-Springs L or R, or if their seals are leaking air

thanks Jim, I need to ask you @ dealer each of "air-springs" is $847, web "belestiens it's $620, should I buy from web the L & R & have a mechanic change for me, it will be cheaper than the dealership, as for them to diagns. it's $112. NOTE NOW IM NOTICING ONLY THE REAR LEFT IS SAGGING & NOT (ANYMORE) THE REAR LEFT? SOMEONE TOLD ME IT COULD BE THE SEAL ON 1 OF THEM GONE BAD? DO YOU THINK OF I CHANGE THE REAR RIGHT SIDE AIRSPRING WOULD MAYBE SOLVE THE PROBLEM?? I WOULD APPRECIATE YOUR RESPONSE!!