Sniffing out a rotten egg from the dozen Is it possible to tell if an egg has gone off? 

I am not referring to those eggs that have been especially brined to look like they've been sitting around for decades, but to fresh eggs you want to use and not feel like being punched in the face with a sweatier-than-thou sock.

Is there a trick to detect if an egg is rotten before opening up the shell?
Yep. Rotten eggs float, fresh eggs sink.

This is because eggshells are porous, so over time water vapour and gases leak out, reducing the egg's mass.

A fresh egg will lie on its side on the bottom of a glass of water. The older the egg, the more it sits up, until it's floating.