Test you water and see. But also tests have to check for something, so there's no testing for everything. Rare contaminants can slip though standard tests because every additional test adds to cost.
It's possible to make water yourself better than what the tap will supply, but it's completely on your shoulders for what comes out and it's likely to be very expensive relative to turning the faucet.
With a few notable exceptions most water contamination comes from the customer's side of the main.
Also, remember that while generally your sense of taste is generally a good indicator of quality, ethylene glycol and lead are both delicious.
