If fertilized, you'd be able to determine that within about a week after the egg is laid. For a relatively small amount of eggs, you can do the inspection with a small flashlight. It's called
candeling... just hold the egg in front of a small but focused flashlight in a semi-darkened room. A fertilized egg will show an embryo clearly through the egg shell. Don't mistake a small blood spot which are in a small number of eggs and has nothing to do with fertilization. It takes about 21 days for them to hatch... By the way, here's what to look for:
http://www.homestead.com/shilala/candling.html
Here's some homemade candlers... I use the box one... One can purchase the devices from various sources at fairly reasonable costs... Best of luck!
http://www.urbanext.uiuc.edu/eggs/res25-candle r.html