The North American Industry Classification System (NAICS) is used to classify business establishments by the U.S. government. For example, 44111= new car dealers. SIC is also a classification system that has been widely replaced by NAICS.
Often, databases will use NAICS codes as a means of identifying specific industries by placing it as a choice in drop down menus. Find the NAICS codes through this lookup tool. You will also find a company's NAICS codes on any company profile page in the databases. Sometimes, Googling for the keyword "NAICS" and the desired industry can work. Just make sure to read the description of that code to ensure it is what you are looking for.