Test water hardness with a strip from Lowe's, Home Depot or Amazon: dip it, hold it level about 15 seconds, and match the color to the chart on the bottle. 7 grains per gallon (120 mg/L) or more is hard, and that is where I recommend a water softener; over 10.5 grains (180 mg/L) is very hard. Divide mg/L by 17.1 to get grains. For an exact number, use a certified lab test or your city's Consumer Confidence Report.
You can test your water's pH at home with a strip kit (about $25) or a digital pH meter, and a certified lab test is the most accurate. The hardware-store strip kits only read down to about 5, so below that you need a meter or a lab. Here is how I rank the three tests, how to take the sample, and what your number means: below 7 is acidic, at 6.0 and up plain calcite works in any size tank, and below 5.5 it takes calcite with FloMag mixed in.