A hen will lay her eggs every 2 days. These may be marked with a non toxic felt pen or pencil. You can check the fertility of each egg approximately 5 days after it was laid by holding the eggs in front of a torch beam.
If the egg is fertile you should see little red veins. If the egg is not fertile you could see a few different things: nothing (could mean chick is so developed that it fills the entire shell) or you could see large off centre air pockets. If you are inexperienced it would be best to leave the eggs with the hen regardless as you may have mis-interpreted what you saw.
Read this link for additional information: Can I touch the eggs with my hands?
A good discussion to read before you decide an egg is not fertile: About Candling
If you arent comfortable candling the eggs ....handling them at this stage....you can tell by the colour of the egg. The fertile ones are a solid white..the infertile ones are a creamy pinkish colour.
Fertile egg on the left ...infertile egg on the right ........see the difference in colour ?