I was taught decades ago to open the chamber, look, and stick my skinny finger down there to "feel" the empty chamber. This has been passed onto my children. A firearm is checked every time it is picked up, or accepted.
When I asked my father years ago if I really should stick my skinny finger into that M1 Garand action to make sure, his reply was: "God gave you ten fingers for a reason. Would you rather lose a finger or cross paths with a 30-06 round?"