You can make a test with calibrated solution and the tap water using similar containers. Then compair at direct daylight the results between two containers.
Just for your info, tetra tests and JBL ones are really bad. I tried many, even Sera tests. The best test that I could verify calibration was Salifert NO3 test, sensitive up to 0.2 ppm NO3, and with a scale of 0-0.2-0.5-1-2.5-5-10-25-50-100 ppm
Give them a try with reference solutions and you'll see how they fool you
JBL detects 1/2 to 1/4 the actual value
Tetra often detects 20ppm even at 0 ppm and is not sensitive to low values
Sera starts at 10ppm only then big steps
For most of what we use a test kit for, you don't need the accuracy of a digital scale. You can use ordinary kitchen equipment, by following this: Calibrating Test Kits - for non-Chemists This isn't good enough to satisfy someone who wants to know "exactly" how much nitrate is in the water, but most of us just want to know that if the test kit says we have 40 ppm, it isn't really 1 ppm or 10 ppm.