The salmonella that eggs get inside them comes from the chicken who laid the eggs.
For eggs to get salmonella the chicken has to be infected with the salmonella and if not then the eggs won't have salmonella.
It can be hard to tell if an egg has salmonella because there's no smell or look to the egg that contains salmonella so it's really impossible for a consumer to see or know if the egg is infected with salmonella.
The good news is though that if you do have salmonella infected eggs the eggs are still safe to eat as long as you fully cook the eggs.
Make sure to cook the eggs all the way so that they are not runny and when you cook the egg fully the cooking of the egg kills off the salmonella bacteria.
So you won't get sick from a fully cooked egg even when it contained salmonella.