You can tell what the fertility rate is for a particular hen (or pen) by looking closely when you break the eggs open for use in omelettes or muffins or whatever -- see sticky at top of this section for a good thread w/pics to help you see the difference between fertiles and infertiles. However obviously once you've broken the egg open you can't incubate that particular egg
The only way to tell *for a particular egg* is to incubate it and see if something starts to develop.
I usually check for "bullseyes" when I'm cracking eggs for eating and then I'll know that pen is fertile. If you don't want to waste the egg, then just set it in the incubator for about a week and candle it to check for development.