Arsenic has no taste, no smell, and no color.
What it is
Arsenic is an element that dissolves naturally out of bedrock, and parts of the Marshall Sandstone running through the Thumb and mid-Michigan are known for it. It is reported in micrograms per liter or parts per billion, and the EPA drinking water standard is 10 micrograms per liter. It exists in two forms, and which one is present usually decides whether a given treatment works.
What it usually does
Nothing you can detect. It does not stain, it does not smell, and it does not change the taste, which is the whole reason it is tested for specifically rather than noticed. Health effects are associated with long-term exposure, and that is a matter for a doctor and the health department rather than for us.
What it does not prove
A neighbor's result is not yours. Arsenic varies sharply between wells drawing from different depths on the same road, so a number from next door usually settles nothing about your well. A general mineral panel also usually does not include it, which is why people who assume they have tested for it often have not.
When to test
Usually worth testing when buying a house on a well in a known area, and periodically after that. Use a lab that reports to the standard, and ask for the speciation if treatment is being considered, since the form changes what will hold. If a result comes back above the standard, the county health department is the place to call.
See what this usually means for your water.
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