Unfortunately, the only way to determine fertility before incubating an egg is to crack the egg open and examine the germinal disc -- which would, of course, render the egg useless for incubation. If you are hatching from your own flock you can sacrifice a few eggs over several days of laying to judge the overall rate of fertility (test eggs from different hens if you are aware of who is laying which) and then when you feel fertility is at a good level set some eggs to hatch. Alternatively, or if you are incubating eggs from another source than your own flock so supply is limited it is fairly simple to set the eggs and candle after a few days to check development.
