As a former chemist I can say that the best way to measure the concentration of any mineral that is dissolved or suspended in water is AA spectrometry. The liquid is injected into a flame and the absorption of specific light frequencies is measured which gives very accurate concentrations. Very Expensive, $hundreds per sample.
Another way of determining the metals content of a colloidal solution is to measure the specific gravity.
Starting with distilled water which by definition has a specific gravity of 1.00, as greater silver ( specific gravity 19.3 ) is suspended, the the specific gravity of water increases.
So 1 liter of pure water will weigh 1000 gm at 32 deg F. At 20 ppm CS should weigh about 1000.4 gm, 200 ppm should weigh about 1003.86 gm and 500 ppm should weigh 1009.65 gm. Yes, you will need a very accurate scale with good resolution for this.
Also, you can get a number using electrical resistance. Distilled water has a very high resistance but this decreases with silver content ( as well as most metals ). However, the issue is that ionic silver decreases the resistance far more than colloidal.
Neither of these will measure particle size which is very important.
For measuring the actual particle size it is best to use an electron microscope. Very expensive!