You'll want to mark what side was up when they were laid. Usually you can tell just by looking at an egg if it's fertile or not. Is the eggs bright white & solid looking? Here is a pic of some fertile eggs so you can have an idea of what they look like.
Also you can candle the eggs. In a dim room, use a small LED light & hold it up to the egg. If it's fertile, it should have a donut shaped circle of veins, what's called a cheerio. It might take a day or two for the cheerio to be visible though. If you search around a bit, I know there are some really good pics on here that show what a good candled egg looks like.