You can determine whether an egg is fresh by conducting a simple experiment. Fill a large glass full of water and gently add an egg. Fresh eggs will sink to the bottom of the glass, while older eggs will float. This doesn't mean the older egg is inedible. Before using it, smell the shell to detect any off-odors. If it doesn't smell, crack it open into a separate dish, check the appearance and smell the raw egg. Discard any eggs that look unusual or have an odor. As eggs age, an air cell begins to form inside that makes the egg more buoyant.
Place a fresh egg gently into a large glass of warm water. It will sink because it is denser than the water. Remove the egg and gradually add and stir in pickling or Kosher salt until you can't dissolve any more salt in the water. Put the egg gently into the water. It will float on or close to the surface because the salt increased the density of the water to the point that it had a higher density than the egg. This is the reason it is easier for you to float in the saltwater of the ocean.
Fill a large glass half full of warm water. Add pickling or Kosher salt until you can't dissolve any more salt into the water. Be careful not to let the two types of water mix as you gently pour fresh water down the inside of the glass. Carefully put the egg into the water. It will sink halfway down and stay suspended where the fresh water and salt water meet. The denser salt water stays in the bottom half of the glass.
Pour salt into a large glass until it is 1/5 full. Very slowly, add water until the salt is barely wet and makes a cohesive salt layer at the bottom of the glass. Fill the rest of the glass with cold water letting the water flow gently down the inside of the glass so the salt layer remains undisturbed. Add the egg and watch it sink. Cover the opening of the glass with plastic wrap and a rubber band. Observe the egg daily over several months and mark its rise on the glass. The salt layer gradually mixes with the water immediately above it, causing the level of denser water to rise and carry the egg with it.