How do you know when a guppy is giving birth?

Fish lay eggs. They don't bear live offspring.

SECOND OPINION: Yes, but only half of an answer. Females about to lay make a "nest" in small rocks and lay their eggs there, so after the goofy male fish comes and fertilizes them with his cloud of sperm in the water, which then penetrate eggs and then the baby fish develops inside of that egg., living on fluids in the egg itself. With any luck at all, many of the now-sticky eggs will attach themselves snugly down between those rocks to prevent their being eaten by other not-so-nice hungry fish. The female mother fish hovers over her "nest" to protect her eggs and tiny hatchlings until they are big enough to quickly get out of harm's way. VERY few of the tiny baby fish live long enough to reach maturity.

THIRD OPINION:

Not all fish lay eggs. Some are livebearers, which means that the baby fish develop inside of the mother, and then the mother gives birth to the live fish. Fish such as Mollies, Platies, and Guppies give birth to tiny baby fish that swim around the tank right after their birth.