First do the sniff test. If they smell off, toss. Gently!!!!!
The float test does not tell you if they are good or not, but it is a good indication for age. As an egg gets older, it loses moisture through that porous shell. When it it fresh, if you put it in a bowl of water, it will lay on the bottom. If it is a little older, the air sac on the fat end gets bigger and it will stand on end. When it has lost enough moisture, it will float on top.
An older egg does not mean it is a bad egg. Unless bacteria get inside, it won't go bad, even if it is left out for months. But the float test is a good indication of which ones you should be more suspicious of.
Another thing. If the eggs are stored in warm temperatures, even as low as the mid-80's, they can start to develop some if they are fertile. They won't hatch at those temperatures, but they can develop enough for you to see something. Not that those eggs are bad. As long as no bacteria is in there, they are safe to eat. But you won't see me eating a partially developed egg. That's why I always break them in a separate bowl before I use them.