How round is a large American egg?
Down below is a link that may help you.
If the shell isn't too thick or heavily coloured, you can candle the egg. Hold the egg up to a bright light, don't let too much light come round the egg, and you should be able to see the contents. For people incubating large numbers of eggs artificially, there is now a machine that can pick-up heart-beat inside an egg - Egg-buddy.