Your chemical supplier should also supply titration kits for testing concentration. While conductivity meters and flow can help, they're not as accurate and you should use a titration method to check on a risk assessed basis.
When you're deciding where to check and how often, do consider where you dilute your chemicals. If it's in one place you will need to do fewer checks than if you have multiple dilution stations. Also when taking a sample, allow it to run for a minute or so first so it's representative.
These are the kind of test kits they supply: Chemical Titration Test Kits
Depending on the contractor, in the UK at least, some of the suppliers will do this testing for you (depending on how often they're on site as well). Not sure if that's also the case in the states but worth asking.