This flow rate meter is not specifically designed to plug right into a water cooling system, therefore it takes a moderate amount of wiring and soldering to make it ready.  It is a flow meter designed for a wide range of DC power and provides a sensing output.  You have to make it ready for a computer and that involves splicing and soldering several resistors into place.

For my particular testing, I had chosen to prepare it for a computer fan header.

The parts I puchased at my local Radioshack for $99 a package.  They are 1/2 watt resistors in the flavor of 100 ohm, 150 ohm, and 2.2K ohm.  The fitting couplers shown below I fabricated myself to convert the 3/8" BSPP male threading on the meter to a 1/4" BSPP female threading to accept standard 1/4" BSPP or G1/4 barbs  These can also be purchased at mcmaster-carr.com in a brass coupler.

This is the results of my wiring and soldering.  The intent here is you want 5V going to the meters power line and 350 Ohms will reduce 12V from the fan header to 5V.  The 2.2KOhm resistor is just part of the wiring diagram from swiss flow.

The most scientific way to determine hydraulic resistance of water cooling parts is to test pressure drop. Pressure drop is a measurement of pressure loss across a block that varies with flow rate.  This is basically a measurement of energy loss, and directly influences how much flow rate you will have.  A centrifugal pump (What all water cooling pumps are) by design moves the most amount of water with the least amount of resistance:

• Dwyer Digital Manometer 475 Mark III - Accuracy .5% of Full Scale.  Range 0-200" W.C., Resolution .1" W.C.
• King Instruments 7520 Series 0-5GPM, 250mm scale - Accuracy 2% of Full Scale.  Range 0-5GPM, Resolution .1GPM (can be interpolated to .02GPM)
• Water Source - Household water pressure - 50PSI at >5GPM - Because flow rate readings are instantaneous, household tap water and water pressure are a good and powerful source for pressure drop testing.