Rust-colored marks in sinks, tubs, toilets or on laundry.
The stain's texture (powdery vs slimy), whether it's whole-home, and an iron test that distinguishes dissolved from particulate forms.
Slimy deposits raise the bacteria question — an ordinary iron filter won't solve that.
Dissolved, particulate and bacterial iron need different equipment. The test costs far less than the wrong filter.
Oxidizing filters, aeration or chlorination each fit specific confirmed conditions.