How do you know if it is a baby turkey or a peacock?

Answer 1

You appear to be referring to the fan possessed by tom turkeys and peacocks, displayed by males of both of their respective species.

Baby turkeys, toms and hens, range from white to yellowish, or primarily brown with dark and light stripe like markings. Tom chicks do not have a fan. Tail feathers of turkeys do not develop until later adolescence.

Essentially the same is true for fan development of peacocks.

While pea chicks do look very similar to turkey chicks, by the time the fan develops, males have transformed to display deep blue and greenish breast and head feathers with plumes on the crown of the head, and prominent fans with eye-like markings.

One cannot easily mistake mature tom turkeys with mature peacocks.

If you are specifically referring to the chicks of both species, they are not easily distinguished from each other by appearance.

If you are seeing the birds in the wild, in the US, likely as not the birds are turkeys, but this will be obvious by the presence of the mother or hen.