Know the Industry Classification
Every industry has a classificaton developed by the US Office of Management and Budget. Currently, companies are classified using these two systems:
NAICS (North American Industry Classification System)
SIC (Standard Industrial Classification)
NAICS is an update of SIC and provides more detailed industry breakdowns. Both systems are still in use, so knowing both codes will allow you to more easily access information in a variety resources.
Find Corresponding NAICS for an SIC (& vice versa)
Use these Concordances from the NAICS website