Hi there. the best way to do this is by a method called 'candling'- this is the process of looking into an egg by means of a narrow beam of light through the shell(eg.torch), preferably in a darkened room. You can ascertain if the egg is fertile after about 7 days of incubation, when a spider shape of blood vessels can be seen if fertile. White shells are quite easy to see through, this becomes more difficult with dark brown or green eggs.
Andrea
Bird Veterinarian
15 years in companion animal practise, BVM&S degree Edinburgh university