I just bought a new RO/DI unit and I also bought an electronic tester. It has an input line and output line. Which you could put the input line before the unit or before the membrane. It's pretty cheap and is so easy. Turn it on and read it. I'll see if I can get a line for ya. Plus, I've always been told if you replace the membrane, replace the other filters at the same time. If not, the membrane will get dirty alot faster without the new filters in front of it.
http://cgi.ebay.com/DM-1-Inline-Dual...sid=p1638.m118