Use magnetic viewing film. It is a thin sheet containing tiny magnetic particles or flakes that turn to face the field beneath them. Laid over a magnet, it shows where poles are and where the field changes direction, as a pattern of light and dark or colored lines.
For coded magnets it is the quickest inspection tool there is. You can confirm a part has the right pattern, check that a mating pair is matched and oriented correctly, and spot a part that was magnetized wrong or partly demagnetized. A plain disc shows one uniform area; a patterned magnet shows rings, stripes or a grid of regions.
Correlated Magnetics Research sells high-resolution color viewing film in its catalog in two sizes, 100 mm by 75 mm and 200 mm by 300 mm, and includes a piece of green viewing film in its Polymagnet Demo Kit.
Film shows where poles are, not how strong the field is. For strength, use a gaussmeter or a pull-force test rig. Keep film away from strong magnets when stored, and do not slide magnets across it, which can scratch it.
WholeMagnetics, 2026-10-04. Polymagnet facts are taken from Correlated Magnetics Research’s public pages, listed below; vendor performance figures are CMR’s own.