An interesting idea of using the camera sensor, sensitive to beta, gamma rays or higher frequencies provide an accurate reading of radioactivity. It's not as fast as other specialized equipment but this Android and iPhone application called Radioactivity Counter is capable of being compared with them after a certain period of data acquisition. Readings below a power threshold are difficult, but nonetheless it's perfectly enough to identify sources of radiation that could be problematic for health.
The application emits beeps as the usual Geiger counters so if you find an area where the sound is more constant, it's for sure a source of radioactivity.
Some people say that ignorance focuses fear and the knowledge about radioactivity is no exception. Like to travel by plane, the statistics are low and even in areas where natural radioactivity occurs the chances continue being scarce. The contact with radioactive sources might be, however, a cause of health issues of extreme importance and then it's recommendable to avoid them, specially if it's as simple as to place an object at a certain distance.