Use the glass test first: if cloudiness clears upward within seconds, tiny air bubbles are the likely cause. Persistent particles or discoloration deserve further investigation.
The harmless cause: air bubbles
USGS explains that cloudy or milky-looking water is often caused by tiny air bubbles released as pressurized water reaches the glass. The bubbles rise and disappear.
When cloudiness is actually sediment
If particles settle to the bottom or remain suspended, the issue may be sediment, scale, plumbing debris or a recent utility disturbance.
When testing is useful
Persistent discoloration, unusual odor or a sudden unexplained change can justify contacting the utility and using a targeted water test.
A lab test is more useful than buying a filter blindly when the cause is unknown.
Treatment depends on particle size and source
Sediment filtration can help with particulate matter, but it will not solve dissolved contaminants or plumbing corrosion by itself.