There are several ways to determine if an egg is fertile and viable for incubation:

1. Candling: Candling involves shining a bright light through the egg to see the embryo and determine if it is developing properly. Candling is typically done on day 7 and day 14 of incubation. A fertile egg will show a dark spot or veins within the egg, indicating the development of an embryo. An infertile egg will appear clear or with a small white spot and will not show signs of development.
2. Egg fertility testing: You can purchase an egg fertility testing kit from a farm supply store or online. This kit involves a simple test that detects the presence of a specific protein in the egg that indicates fertility.
3. Observation: Observe your hens and roosters to determine if they are mating and if the eggs are being fertilized. If the rooster is mating with the hens regularly and the hens are laying eggs, there is a good chance that the eggs are fertile.

It is important to note that not all eggs will be fertile, even if the rooster is mating with the hens regularly. Also, not all fertile eggs will develop into healthy chicks. Proper incubation conditions and regular monitoring are important for ensuring a successful hatch.