What a Geiger counter tells you
A Geiger counter, sold here as a radiation dosimeter, measures ionising radiation in the area around it and reports a dose rate you can compare against normal background levels. People use them to check building materials and stone benchtops, test imported goods, or simply establish what the background reading is where they live and work.
The practical differences between models are detection range, how quickly they settle on a reading, and whether they log measurements over time. If you are testing objects rather than monitoring an area, prioritise a fast, stable reading over logging.
Related
For the rest of the detection range, from hidden cameras to trackers hidden on a vehicle, see counter surveillance.
Common questions
What is a normal background reading? It varies by location and altitude. Take several readings where you live to establish your own baseline before comparing anything against it.
Can it test food? These units measure surface and ambient radiation. They are not laboratory instruments.