Part # TDS01
These injection-molded testers are built with a rugged “one piece” splash-proof casing and are supplied complete with calibration solution, batteries and screwdriver for calibration. Color may vary. 

Reverse Osmosis Membranes have an average life of 2-4 years. 
I have seen them go bad in only 1 year. 
Testing the water is the only sure way to know if it's time to replace the RO Membrane.

With a TDS Meter you can always check the quality of your reverse osmosis water. 
To do this, first check the reading of the source water from your normal faucet. 
This number will be high ( 100 to over 1000 ppm - average about 300 ppm ). 
Then test the water coming from your reverse osmosis faucet. 
This number should be 90 -95 % less. Generally less than 100 ppm ( typical less than 50 ppm ). 
If the reading  is not at least 75 % less, then you should change the membrane. 

Some of the features:
RANGE: 0-999 ppm. Comes with Calibration Solution ( 800 ppm potassium chloride ). Batteries normally last years ( replacement available at Wal-Mart and Radio Shack ) and an auto shut-off feature will shut the meter off after 5 minutes of non use, preserving the battery life.