TDS (Total Dissolved Solids) is a good way to measure the purity of the water in terms of "dissolved" solids as well as ionic and elemental salts (ie. sodium, chloride, nitrates, phosphates, magnesium, sugar, dirt, etc.).

Just be careful not to rely on the TDS meter to tell you if the water has absorbed any BTEX compounds (Benzene, Toluene, Ethyl Benzene or Xylene compounds found in gasoline) or other organic solvents, pesticides, ammonia or herbicides as it will not show up on a TDS meter.