Using test kits that require titration to a visible color shift can be somewhat problematic. Any two analysts can get consistently different results based on each individual's interpretation of where the endpoint is. From the reefkeeping perspective, that's not usually a problem so long as the reefkeeper performs the test consistently.
However, the best way to estimate accuracy (as opposed to precision, which has to do with getting the same, but not necessarily correct, answer repeatedly) is by testing a standard solution. In the case of the Salifert alkalinity test, a standard solution is provided with a known value, so you might choose to test this and see how close you are.
Calcium is a bit more problematic. If you've an accurate scale (down to the gram) and a way to make accurate volume measurements, you can prepare a standard solution by means of serial dilution that will be accurate to +/- 10 ppm.
But without the equipment, perhaps the second best way to compare two test methods for calcium would be to make a batch of seawater with a consistent salt mix. For example, Instant Ocean when freshly mixed to a specific gravity of 1.026 should test at about 400 ppm Calcium. Reef Central members consistently report that Reef Crystals, when freshly mixed to 1.026 specific gravity, has a calcium concentration of about 490 ppm.
