I`ve been raising and breeding fowl for 51 years and have always believed that it is imposible to determine the fertility of an egg by looking at it. While it is true that the egg must be opened, thereby eliminating any chance of hatchability, the following link illustrates that fertilization can be determined and will therefor be likely in other eggs from the doner pair. My thanks to MissPrissy for calling me on the statement I made to the contrary. Old dogs can indeed learn new tricks.
http://www.thepoultrysite.com/articles/160/breakout-analyses-guide-for-hatcheries
http://www.thepoultrysite.com/articles/160/breakout-analyses-guide-for-hatcheries