About 1 week after incubation (i.e. hen sitting on eggs without leaving them) to see if they are fertile or not. By two weeks, you should know without a doubt! By week 1, you should be able to see the formation of veins forming with possibly a red dot in the center.
If some eggs are fertile and others aren't, leave them all be! The infertile eggs can help provide warmth to the fertile eggs and/or chicks, as well as support.
If all are infertile, allow the hen to sit on the eggs for at least 3-4 weeks before removing them or allow her to abandon the eggs before removing them.