This is awesome, I've always wondered how to tell the difference because there were a few eggs I'd eaten that made me question whether or not they were fertile! I have a question too - what could be a little, hard brown spot near the blastodisc/blastoderm? I worried it might be the very beginning of a little growth but I have no clue! Looks sort of like this:

If you have any idea, let me know! Thanks!