Crack a fresh, unincubated egg into a bowl in good light. If you have sharp eyes, you will see that a fertile egg has a faint bullseye, the fertilized blastoDERM, on the yolk, while for an infertile egg, it will just be an irregular spot, a blastoDISC. If you don't have sharp eyes you might make use of a magnifying glass--or ask someone to help. Remember to check the bottom of the yolk; sometimes the way your egg lands in your dish will make the bullseye difficult to see.
Illustration by Ray Yang for My Pet Chicken
There's a problem, though--if you're planning to hatch fertile eggs, surely you've noticed it! The problem is that if you're cracking your egg to see if it's fertile, you won't be able to hatch that egg.
So you get, "Hooray, it's fertile! Boo, it's ruined for hatching!"
So, in order to know what your fertility rate is for incubation, just keep track of how many eggs are fertile as you crack them to cook. Keep a little kitchen journal. Base your fertility rate on what you've observed in your journal. If 8 of 10 eggs are fertile when you crack them to cook with, then probably 80% (-ish) will be fertile when you set them in an incubator or under a hen.
Remember also that the larger your sample, the more accurate your rate will be.