Most professional home inspectors belong to a governing organization or association, in addition to some requiring state licensure.
These organizations provide training, certification, and hold their member inspectors to a Standards of Practice (SOP) when performing inspections.
The Standards of Practice document defines the scope of work that the inspector will perform. It includes:
There 2 major home inspector associations/organizations that most professional home inspectors belong to. Here are links to each organization's Standards of Practice in PDF format.
InterNACHI Standards of Practice