how to find it?

The white thread is OK not a problem. The way to see if an egg is fresh or stale is to place the egg while still in the shell in a bowl of cold water (or even the pot you're going to boil it in). A good egg will sink to the bottom while a stale egg will float to the top. Remember this must be fresh water - if you have salt in there a fresh egg will float too!!

the first thing that you have to do when you buy eggs is look the validate date,to see if they are good,if you buy in a farm or in other place that dont come with the date,always ask the saler when the eggs came and when you buy then open one and see if they are with a bad smeel or blurry..if the egg has one of these things,they are not good,go back to the store and show to the saler..you will not be with a bad product and maybe you will help other people that dont have theses cares to not have health problems eating these eggs:)

I go with how the egg smells. Before refridgeration, people would keep their fresh eggs out on the counter. I even read where they used to pack them in ashes and keep them for up to a year. If the egg is undamaged and does not smell, its probably fine. I keep my fresh eggs on the counter in a basket, but they are farm fresh, I don't know about the ones you buy in a store.