Exactly, but I had someone once tell me to not do it that way so I was kinda left going WTH LOL! I can't think of any logical reason you wouldn't test it post filtered since you'd use that with every single batch.
Except that the filters slowly lose efficiency (assuming moderately hard water). I tested before the RO filter, and use that value with dilution water to predict alkalinity and other ion concentrations. This gives me the option to use pre-filter water diluted with RO to achieve my desired result.
With a THD meter, you can determine a rough efficiency for your filter and compare to the filter's published efficiency. While the ion removal is not absolute across all ions (molecules vary in size and efficiency with RO) you can make a reasonable assumption on the ion concentrations post filter. As the TDS rises, you can adjust the assumption and still be close. Once the TDS rises above 40, you may wish to consider replacing your filters. In my case, sodium and bicarbonate are my limiting ions in my source water (really really bad) and a fresh filter (at 60 PSI) reduces pre-filter TDS over 800 ppm to below 35. I use the estimated efficiency provided by the system (at 60 psi) to reduce those values by 95%). As long as the TDS is below 60 ppm (in my case) 5% of my original ion concentrations get me reasonably accurate mash pH estimations. I use the system efficiency estimates as they are close enough... measured alkalinity with a Gh/Kh kit shows I am close to 18 ppm alkalinity with a new filter, good for building water with minerals... however precision is not a function of those specific test kits.
It depends on what kind of filter you are talking about. If it is a wound (particulate) filter or a carbon block filter or combination of both then it really doesn't matter as these don't effect mineral content (I assume you are contemplating a mineral only test such as the popular Ward Labs tests that so many on this forum have done). If the filter is an iron removal filter then again it doesn't matter unless you pick a test which measures iron. You would, in that case, want to check input and output to see if the filter is doing its job. If a neutralizer is involved then again, both input and output should be checked to see what effect the neutralizer is having on pH, alkalinity and hardness. With softening an input only check will do as softeners are so effective that you can take the input hardnesses (as CaCO3), divide by 50 and multiply by 23 to get the additional sodium (add to input sodium) and call the output Ca and Mg hardnesses 1 ppm as CaCO3 each and be very close though again input and output tests confirm proper equipment operation. If an RO system is involved check TDS as suggested in #4 but test at the input.
In general, test at the source coming into the house. This will give you the best picture as to what you have to deal with. In terms of what to enter into a brewing spreadsheet, for example, given only any input test, you can assume that your on line equipment does what it is supposed to. The exception to this might be the neutralizer. Easy solution there is take it off line when you brew. This makes sense as all it does is add alkalinity and a bit of hardness and except in rare cases you don't want any more alkalinity than you are already stuck with. In those cases you add controlled amounts yourself.
Stephen, what did you end up doing? Did you decide to filter the water and then send it end?
I just ordered a test kit myself and debating what to do. I have a Pentek ChlorPlus Carbon filter for removing Chlorine and Chloramine, although I just found out my water supply is on a new RO system and no longer uses Chloramine.