-check feed water pressure: normally around 200 psi
-Check feedwater pH: I believe around 6.0 - consult the manufacturers recommendations. If the pH is too high, this can cause fouling of the membranes.
-Check fw conductivity to find average - if off average, troubleshoot.
-Check Feedwater for grit, slimes, trash etc. Pre-treatment at minimum should be cartridge filters.
-Check permeate conductivity - should be very low around 5-10 micromhos. If conductivity rises, RO's need to be cleaned.
-Check concentrate conductivity - this helps to identify is the membrane is operating properly.
-Make sure no chlorides are in fw, some RO membranes are ruined by them. Such as using sodium hypochlorite treatment pre-RO.
-check permeate hardness should be <1 mg/L CaCO3

Be sure to trace your pipes, this helps to further your knowledge of where the RO permeate heads to and where the feed water comes from. Always useful when troubleshooting problems.

Also, what application is this RO for?

-Dave