The way to test a membrane is with a TDS meter. A good 10 GPD membrane should be removing 95 to 98% of the incoming tap water TDS. If you don't have one maybe you can borrow a TDS meter from a friend of local club member. You will want and need one yourself eventually so if you end up ordering a new membrane and flow restrictor add a handheld TDS meter to the order.
Fill a glass with tap water and test the TDS then rinse the glass and meter with RO or distilled water a few times and fill it with RO water. You should see a 95+% reduction in TDS between the two. If not the membrane is shot. If the membrane has been allowed to dry out chances are it is bad. Once in use you always want the membrane and if you have an RO/DI, the DI resin to stay wetted. The 75 GPD suggestion is a good one, the 75 GPD Dow Filmtec is the most popular RO membrane by far. Make sure to get the matched flow restrictor and the TDS meter too.
