You can get a detector/dosimeter on Amazon in the $50ish range. I got one a while back and verified it works for increased background radiation at 30,000 feet in a plane and separately with an alpha radiation source. The interesting thing is if you leave it on the background radiation sometimes just spikes for no apparent reason. Maybe something like a cosmic ray hitting the sensor every once in a while?

The one I got is on Amazon as "Geiger Counter Nuclear Radiation Detector, Professional High Accuracy Radioactive Detector Meter Beta Gamma X Ray Data Tester Marble Dosimeter".