You cannot reliably tell if water is safe by looking, smelling, or tasting it. Clear mountain streams carry giardia and crypto the same as stagnant ponds. Treat all untreated field water as contaminated.
What you're actually checking for:
- Biological contaminants (bacteria, viruses, protozoa like giardia) — killed by heat or filtration.
- Chemical/industrial contaminants (runoff, agricultural chemicals) — not reliably removed by boiling; avoid water sources downstream of farms, mines, or industrial sites entirely.
- Turbidity (dirt, sediment) — doesn't make water unsafe on its own but reduces the effectiveness of chemical treatment and clogs filters.
Field triage before treating:
- Prefer moving water over stagnant — lower biological load, not zero.
- Let sediment settle, or pre-filter through cloth, before treating.
- Never assume a source is safe because it's remote or looks clean.
There is no visual test that replaces treatment. Boiling, filtration, or chemical disinfection (covered in "Three Ways to Purify Water Without a Filter") are the only reliable methods.