Most library-sponsored sources of company information and quite a few free websites identify a company's industry affiliation by a government-defined SIC (standard industrial classification) code or government NAICS (North American Industry Classification System) code.
You will notice that diversified companies do business in different industries or industry sectors. Some available reports distinguish a company's primary industry codes from the secondary codes, which represent less-significant parts of its operations. Other resources and reports simply list the various lines of business in which the company engages without flagging the ones on which a company focuses most heavily.
When identifying competitors for a specific assignment requiring comparison of diversified companies, it usually makes sense to select competitors doing business in the specific market segment (e.g. clothing, telecommunications services, computer consumer goods) pertinent to your project rather than comparing of all facets of each company's operations.
Size doesn't always matter when identifying competitors for a specific market segment. For example, a relatively small company in terms of total annual sales such as Paychex can be a major competitor to the payroll services business segment of ADP's operations, although it may not be a major competitor to ADP's other business services.
Industry Classification examples
You can obtain In-depth company reports by Hoover's from the Nexis Uni database.
Sometimes reports from different stock advisory companies within the same database will list a different industry classification and a different list of competitors for a given company. This is where you exercise judgment as to which companies from a peer list make a sensible basis for your comparison of industry competitors.