Hold an egg to the light - no dark specs - feels heavy - fresh. To be doubly sure, place the egg in a jug of cold water.
If it's newly-laid, it will lie on the bottom - fresh. Ideal for poaching, frying or mayonnaise, which you can whip up from my "Healthy Mayonnaise in Seconds" Snapguide.
Egg shells are semi-permeable. After more than a week, air sac present in all eggs, gets larger. Large end will rise slightly at an angle - fairly stale. Best for omelettes, sauces, or cake mixtures.
After about two weeks - vertical. More stale. Ideal for hard-boiled - easier to peel shells for deviled eggs, etc.
Eggs are ok for about 3 weeks in average. After more than 3 weeks - egg floats to the top - bad - discard. A bad egg is quite light in weight and smells pungent.
View from above: A bad egg,
a stale egg,
and a fresh egg.
Tip: Store eggs small end down in air-tight egg carton in the fridge (so that eggs don't absorb any unwanted odour from fridge). Stay healthy and kind and have a nice day! Cheers!