I have a question about eggs and when it is safe to eat them.
I've read online that there are 2 ways to test if an egg is safe to eat or not (without cracking it open):
Float test: Get a massive bowl, fill it with water and place the egg inside. If it sinks to the bottom and lays on its side, it's "fresh" (not old). If it bobs up slightly then it's a few weeks old and if it floats to the top throw it away (too old).
Those sites also say that test is no indication of whether or not an egg is rotten, it only indicates the age of the egg and that eggs can be rotten before they even expire.
Slosh test: Gently shake the egg near your ear. If you hear a distinct sloshing sound, the egg is rotten and should not be consumed.
Is the above true? Are they old wives tales/myths?
I've also read that egg shells covered in crap and blood are not safe to eat because they more than likely have bacteria. Why do most eggs have crap and blood on them?
I've also read that eggs with dark colored spots all over them should not be consumed. Why? Why do most eggs have weird colored spots all over them?
Sometimes when I gently shake an egg, it does not slosh but I can feel the insides moving around (it kind of feels like a big ball inside rocking back and forth) - is that normal? Is it safe to hard boil and consume those eggs?
I guess my question is, what are the guaranteed ways to determine if an egg is safe to consume without cracking it open? And please, if you can provide any official sources that would be great.