Put them in a container of fresh cold water. The higher they rise, the dodgier they are. If they float completely, they're probably not safe. I think as long as they don't float, they're edible.
EDIT:
Researched a bit, and if it rests on its pointed side on the bottom, it's a bit older but fine to eat, if it floats, it's stale. Really fresh eggs should lie sideways on the bottom.