There is no reliable way for the average home hatcher to tell the difference between an egg that will hatch a female chick and an egg that will hatch a male chick. In-ovo DNA testing and spectroscopy are being developed for certain applications, but most of us don't have this technology readily available at home!
Instead, there are lots of old wives' tales out there, most of which have to do with the shape of the egg or which way a string swings when you hold it above an egg. In most cases, you will find that in one area of the country someone will swear that round eggs hatch hens, and in another, it is sworn that round eggs hatch roosters. Each "side" will have people who will tell you how many times they have been able to do this successfully (or that they "know" someone who can do it successfully), but the truth is that no scientific study has ever been able to replicate these results.