The criterion you use will depend on the test method you select. For minimum requirements, look at J-STD-001C, Para 8, "Cleanliness Requirements". I figure that you�d measure the residues on a lot of your current product, measure the res on a lot of your product using NC, and be happy if the res from your NC is lower than your older flux.
Ionic residues are detectable by:
* Resistivity of Solvent Extract (ROSE) testing (Omega meter, Ionograph, etc.). [Eurolanders call this Solvent Extract Conductivity (SEC).] Using your Omega meter is identical to washing your boards in a IPA / DI water solution. That�s a sure fire way to turn your solder connections to a milky white color. Anyone desiring to use an ionic contamination tester should read EMPF [http://empf.org] report RR000013, "An In-Depth Look at Ionic Cleanliness Testing.
* Ion chromotagraphy. This test measures specific ionic residues that can cause failure. It does nothing to check for non-ionic residues that can cause problems. Generally, only large shops can maintain the equipment. IPC [http://www.ipc.org] lists reputable test labs. Dionex [http://www.dionex.com] is the premier supplier. And if you have to ask the cost, you cannot afford it.
* Surface Insulation Resistance. This test actually has a correlation with field failures. [No failures relating to residue, if resistivity is above a certain level. Guaranteed failures, if its below a certain level.] The level depends upon environment, panel design and circuit design [voltage, current, impedance, spacing, indoors vs. under the hood...]. This test should be done on test coupons in an environmental test lab. IPC [http://www.ipc.org] lists reputable test labs. It is rarely done as a process control, since the test takes a week, and is not usually done on production panels. - Concoat [http://www.concoat.co.uk] makes a very good machine. [I believe that Multicore distributes it in the US. Elsewhere, dunno.] - Alpha Metals makes a SIRometer. - ASR Instruments is another reputable supplier.