I've even seen nests cleared of eggs that did not match those belonging to a specific breed. My experience has been that the more feral in nature the breed, the more pronounced this selectivity. Welsummer eggs are much darker than most other eggs. This and the textured speckling on the surface of the eggs easily differentiates them from the eggs of other breeds. Good broody hens, however will accept a mixture of egg varieties.
Artificial incubation is the commercial standard.
Eggs are hatched commercially using sophisticated systems that closely control temperature, humidity, and turning of the eggs to produce excellent hatch rates. These systems are costly however. Hatching eggs economically at home is fun and exciting. It can also be a challenge. Styrofoam incubators with automatic turners and air circulator fans are popular choices for the hobbiest.