Most people would know if they live in an area with hard or soft tap water. If not your water company could tell you. My water company publishes regular tap water test results online. Unless your water is very soft (less than 4dgh) you wont have an issue with a snail as long as you keep up with your water changes.
The api calcium test is for SW
reef aquariums. It specifically says on APIs website the test isnt needed for FW
tanks. I dont know if that means it wont work on FW
though. There are tests for general hardness, but as said you should be able to find that without needing to test. Your water hardness impacts aspects of life apart from keeping fish, like maintaining your water boiler, descaling your kettle, or simply washing your hands.
If you want to add cuttlebone, just drop it in the tank somewhere hidden or pop it in your filter. If you put it in the tank it may float, so you could wedge it down or push it into your substrate a little. When its gone replace it. If you already have hard water it may never dissolve away and you wont notice any difference to water parameters. If you have soft water be aware that cuttlebone will increase water hardness and pH.
You can get calcium into a snail through its diet. Occasionally feed a calcium rich veggie like zuchini or cucumber slices.