At minimum, follow public-health guidance for annual bacteria, nitrate, TDS and pH testing, then add chemistry relevant to your local geology and symptoms before buying treatment equipment.
The annual minimum
CDC recommends testing private well water at least annually for total coliform bacteria, nitrates, total dissolved solids and pH.
Private wells are not regulated, treated or monitored like public water systems, so the owner is responsible for routine testing.
Add chemistry that affects treatment design
If you are buying equipment for stains, odor or scale, add iron, manganese and hardness. Sulfur odor may justify hydrogen-sulfide investigation. Local geology can make arsenic, uranium, radium or other contaminants relevant.
A state-certified laboratory is preferable when health-related decisions depend on the result.
When to retest sooner
Retest after flooding, well repair, noticeable taste/color/odor changes, nearby land disturbance or a new contamination concern.
Pregnancy or a child moving into the home can also justify more frequent review of water safety.
Translate the report into a treatment train
Once results are known, match each measured problem to a treatment technology. Sediment, iron, hardness and bacteria often require different stages.
Do not buy a “whole-house filter” until you can state exactly what it is expected to remove.