Well you can't tell from the outside. So what you have to do, is go with an over all average fertility. Say over a week, you cook 12 eggs. 9 of the twelve have the bulls eye and are fertile. 9/12 = 75% fertility. Which give or take, if you set 10 eggs, 7-8 of them should at least begin to develop. 2-3 would not. Kind of sort of, thing is, there is no way to tell 100% if eggs are going to turn into chicks. Hence the old saying, "Don't count your chicks before they hatch!"
Before you go to set eggs, check them. If you start getting a high percentage for a couple of weeks, that is about the best you can do. If you have a low percentage, which I think that is what you are afraid of with just one rooster, you may have to make some changes. However, the best thing to do, is keep track, week after week, for several weeks and you should have a good idea where you are.
Mrs K