I've had my RO filter (kent RO35 HiS) up and running for a month when needed. Any tips on making sure the water is correct as far as testing it? And what’s the proper maintenance when it's not being used?
Get a TDS meter or ask you LFS to test it for you. When not being use for extent amount of time, some manufacture suggest you keep it (the RO membrane) moist at all time, and put in a refrigrator to keep down the growth of bacteria.
As for storing the filter, I think the main issue is to not let the membrane dry out. When I've stored mine I've just unhooked it and left it full of water. It's been stored like that for months and I've not had any problems when I started using it again.