The difficulty with this question is that the contaminants which make people ill give no warning at all: coliform bacteria, nitrate and arsenic are colorless, tasteless and odorless. So the signs are not in the water, they are in the pattern of who is unwell, when, and whether it tracks the water rather than anything else.
The patterns worth noticing
Recurring stomach upsets, diarrhoea or nausea in the household with no other explanation, particularly if they affect several people rather than one. Symptoms that ease when the household is away and return on coming home. Visitors becoming unwell after staying while residents, who may have partial tolerance, do not. And illness following a specific event such as flooding, work on the well, or a period when the property stood empty. None of these proves the water is the cause, and all of them justify a test.
Why nobody else will find it
The EPA notes that private wells are not federally regulated under the Safe Drinking Water Act, that around 15 percent of the US population, over 43 million people in more than 23 million households, rely on them, and that a USGS study of 2,100 private wells found about one in five carried at least one contaminant above a human health benchmark. No utility samples your well and no report arrives, so a problem persists until the owner tests for it.
What to test, and for whom
Coliform bacteria and E. coli first, since that is the contaminant most likely to cause acute illness and the one that changes fastest. Nitrate next, which is particularly serious for infants under six months and is a reason to test before a baby arrives rather than after. Then arsenic, lead and, depending on local geology, radon or uranium, all of which cause chronic rather than acute effects and give no symptom that would ever prompt a test.
What to do meanwhile
Boiling handles bacteria, viruses and cysts and is the right interim measure while awaiting a bacteriological result. It does not help with nitrate, arsenic or lead and concentrates all three, so it is the wrong response if those are the concern. Use bottled water for infants and for anyone vulnerable until you have results. Then correct the wellhead before fitting treatment, since bacteria usually enter through a failed seal. Our well filtration guide covers the treatment sequence.
Questions people ask about signs your well water is making you sick
Can I tell from looking at the water?
No. Coliform bacteria, nitrate and arsenic are all colorless, tasteless and odorless. Only testing answers the question.
What pattern suggests the water?
Recurring unexplained stomach upsets, symptoms easing away from home, visitors becoming unwell, or illness after flooding or well work.
What should I test for first?
Coliform bacteria and E. coli, then nitrate, then arsenic and lead, plus radon or uranium where local geology warrants it.
Should I boil the water in the meantime?
For suspected bacteria, yes. For nitrate, arsenic or lead, no, since boiling concentrates them. Use bottled water instead.