The larger they are the easier it gets. Pretty difficult as hatchlings, if they grow quickly you can sometimes tell when they're two or three months old. Sometimes hatchling/juvenile geckoes are wrongly sexed even by some of the most experienced gecko folks in the country. Sometimes you'll get a hatchling which will be distinct enough to give its gender away.
I sex my Bynoe's Geckoes as eggs and never get them wrong.