When I was a child, I was witness to a dowser or water witcher.  I believe it can be done, my husband doesn't.  I think 90% of it is actually in the belief, sort of like The Secret.

On a yahoo list I belong to, the subject of too many roosters and not enough pullet chicks came up.  The following is a response by one of the listers.  I have never heard of this before, but it might be fun to try!

Use something made of brass like a key.  Tie a string to the key, hold the egg in one hand, suspended by the string in the other hand directly over the egg (very still) and then stand very, very still. (key does not touch the egg).
The key will start to move and swing. If is swings back and forth the egg has a pullet. If the key swings in a circle the egg has a rooster.
We would mark the eggs with the male or female sign and put them in the incubator. Sure enough...it worked.  The next 12 years our percentage rate of correctly sexing the eggs was around 90-95%!!  It is simple and exciting.

What the hay!  Give it a go!