How does scientists discovered DNA replication and how they prove it?

Question

Through microscope? But wasn't it too small to be seen under microscope?

Answer 1

It was too small to be seen under a microscope.

DNA replication was guessed at after the structure of DNA was determined. The structure of DNA was puzzled out by a number of scientists starting with Rosalind Franklin in 1952. At that time we couldn't directly observe the structure of molecules but we could see how they scattered X-rays passing through them and infer the structure from the scatter.

Though more detailed analysis of X-ray scattering, Crick, Watson, and Wilkins were able to pin down the specific configuration - the famous double helix.