I just accidentally dropped one of my fertile Campbell eggs and cracked it (someone came by to buy some eggs, so I had them out showing her the eggs from all the different breeds), so I'll try and get a picture of it quick and post it here for you to compare.
If the white dot is just a solid white dot, then it's not fertile. If it looks like a bulls-eye or a "halo" instead of a dot, then it's fertile.
The picture isn't the greatest (and it is
really hard to get a good picture of eggs, the way they glare), but it
could be fertile. I don't think it is based on the picture, but I don't know what it
really looks like to the eye.
EDIT: Here's one of my Dark Campbell eggs that
is fertile (the "bulls-eye" is in the lower center of the yolk):
Here's a slightly better picture (same egg) - I fiddled around with the contrast and brightness levels to make it a little clearer and drew in some little arrows:
I wish the ring was a little bigger around - this isn't the greatest example, I've seen some that would have made much better pictures if I'd thought to take a snapshot quick, but I don't want to go breaking open any more of my fertile eggs!