Test kits are acid titration kits. As solvent evaporates, the acids in the flux become more and more concentrated. The kit is used to determine the acid number of the flux. The test is a simple drop test with an indicator for the endpoint (similar to a swimming pool test). You count the drops and then go to a chart. From the chart you determine how much thinner to add to bring the acid number back down to spec. Traditionally flux control is maintained by measuring the density of the flux and adding thinner as the flux thickens. This is a good method for fluxes with high solids contents. A slight change in density corresponds to a small change in the flux. The no-clean fluxes have low solids percents (typically 2 or 3 %) so a small change in the flux density could reflect the change in solids of 25-30%. This would put the acid level unacceptably high. The test kits are more accurate than using density measurements as a way to monitor the flux.
Source: Kester Solder Co.