Question

How could ancient people know that the earth has completed one revolution as they use to assume that the earth was flat?

Answer 1

after coming next monsoon...

Answer 2

No, ancient people were master scientists.
They knew very well that earth is round.
Thats why they named the study of earth as geography which is called Bhoogol in hindi ( meaning round earth).
With the arrival of next monsoon, they used to declare that 1 year has passed and earth has completed its respective one revolutuin around the sun.