All are fertile. If you see a bullseye or donut, it's fertile. Single smaller and whiter spot is infertile. Once in a while I have seen one that wasn't obvious, but this is one thing that is usually clear and obvious.
There have been previous bad years with various strains of avian influenza, but it has been a while since it has been this bad. it is very difficult to make a strain against avian flu because there are so many H varieties and N varieties that when you put them together leaves you with dozens and dozens of combinations. Some just kill domestic birds, while others make people slightly to fatally ill. We have been leading up to a bad year for a few years now. Hard to say when the big pandemic will come.
If I were free ranging my birds, I would stop if humanly possible. especially if your flock is near a body of water where wild waterfowl like to visit. There is no point for hysteria yet, but we should all be informed and vigilant about are chickens and ducks and turkeys. If you have any unusual deaths, especially multiple deaths, the best thing to do is keep them bagged and refrigerated but not frozen, and contact your nearest veterinary school diagnostic lab or state poultry association about getting the bodies tested. You could also get a full necropsy done any time this happens to figure out what's going on with your birds including respiratory disease like mycoplasma.
Indiana offers free avian influenza screenings, and other states may do so as well. Check with your state poultry association.