For an easy "ballpark" look you can do a qualitative comparison. Take a piece of tin foil and put a drop or so of your water you use to water your plants on the tin foil. Next, put a drop of the well water on the tin foil. Heat the tin foil up a bit so the water evaporates (an elective stove on the lowest setting should do; if you're using gas, you can set the tinfoil in a pan and do it.) Presumably, there will be little to no residue left where the distilled water was, if the well water leaves a whitish circle on the tinfoil, then you've got some investigating to do. You've evaporated all of the water, so only the dissolved solids are left. The tinfoil just makes the white spots easier to see.
Note, even if there's no white circle, you still should do some investigating.