The first thing to do is look on your water provider's website for hardness or total hardness. You need a number and the unit of measurement. If it's not there, you could phone them to ask, or take a sample of tap water to a fish store and ask them to test it for GH (though in this last case, if they use test strips, some of them only measure up to 180 ppm aka 10 degrees, or dH).
The reason we ask for numbers is that words can be misleading. My water company gives my hardness as 'slightly hard' but the number is 5 dH, which is soft in fishkeeping terms.