This is what we get hammered with in out Canadian firearm safety classes. This has to be burnt in your mind and has to become muscle memory every time you touch a firearm or you will fail. Sadly some of us tend to forget about it.
PROVE it safe:
Point the firearm in the safest available direction.
Remove all cartridges if any.
Observe the chamber.
Verify the feeding path
Examine the bore for obstructions
Of course it doesn't apply for any kind of carry or home defence but it always apply when handling or cleaning a firearm