I'm actually incubating my first round of eggs today and we're trying to get an 8:1 ratio of hens to roosters. I found a paper from the early 1900s that documented the old way of sexing eggs.

After the egg is laid and has cooled off, a bubble forms in the wider end of the egg. If you candle the egg the bubble will either be centered in the end of the egg if its male, or the bubble will be offset and not touching the end point of the egg at all.

No clue if it will actually work but interestingly enough we did get roughly a 50/50 split from our chickens. I guess we'll find out in about 9-12 weeks if the trick actually worked!