Layout validation can be thought as a simple task as powerful tools are available to check the layout versus the schematic and to simulate the layout including parasitic components. But this idea is not completely true:
The first step of layout validation, even before layout starts is validating the floor plan.
Floor plan is defined by:
This is the well known DRC - LVS - LPE verification trilogy
Once every leaf cell is validated, the validation can continue upwards to the TOP cell