Most people know when you deal with one company, you could end up dealing with another too. That second one could be the parent company, usually a huge business that acts as a corporate umbrella for its subsidiaries. Publicly traded subsidiaries will often list the parent in financial earnings releases or official SEC filings. You can also find them through online searches at places like Hoovers.
Check Financial Reports
Publicly traded subsidiaries have investor relations pages on the official company website. These pages include links to quarterly and annual financial results and the 10-Q or 10-K reports, which are official SEC filings. It might take some digging through the company's information to find a straight answer. For example, Crown Media Holdings doesn't come right out and say it's connected to Hallmark. However, it clearly has some relation to that greeting card giant based on the frequent mentions of the brand on its investor relations page and financial releases. Turn to the 10-Q report, and you'll see it spells out that Hallmark Cards, Incorporated is Crown Media's parent company.
Use Hoovers
The free online search tool at Hoovers allows users to search for the name of public or private companies. The results show a profile of the company with a background paragraph that includes the name of the parent, if applicable. For example, running a search on Janssen Pharmaceuticals shows Johnson & Johnson as the parent company.