How can a geological time scale best be reconstructed?

You'd want to examine homologous structures in fossil remains. This can give a geologic time scale of evolution within certain fossil groups. Without fossil remains a person would not be able to consider the majority of life that has existed on Earth but is now extinct. Fossil evidence can give a general timeline for the common ancestor or origin or homologous structures as well as how they have changed since then.

Hope this helps

Where are they though?