This has been answered in more depth before but the short answer is "they didn't." Naturally radioactive sources on Earth that have existed during human times are so subtle that you cannot tell they are radioactive without specialized instruments (or at the very least, photosensitive films).
The closest one comes to people even being aware of the effects of radiation is a knowledge that people working in uranium mines are more likely to get lung diseases than regular people, which was observed in the 16th century. Even the association of this with specifically lung cancer came in the 19th century.
To create sources of radioactivity that are radioactive enough to have special effects observable without specialized instruments (e.g. glowing or hot or even just dangerous) requires knowledge that only was available in modern times (either isolation of radium, or the use of particle accelerators or nuclear reactors to create artificially radioactive elements).