Look at the name of the company on any product or on any official correspondence or public display. The law requires corporations to identify themselves as such by using a suffix such as "Inc," "Incorporated," "Corporation," or any of a number of other synonyms for the word "corporation." Likewise, limited partnerships, which are the entity type of PTPs, must identify themselves with name suffixes, such as "Limited Partnership" or "LP." This is the most obvious way to distinguish between a corporation and a PTP.
Identify a state where the company has a business presence, if the business entity type is not obvious from the name of the company. Look at products, correspondence, marketing material or the company's website to find any state address for the company. Corporations and limited partnerships must register with a primary state that serves as a home base and must also register with every state in which the company does business.
Go to the website of the secretary of state or comparable business registrar for the state you have identified. Access the business section, often called the corporations division, of the website. Look for a link to the state's business entity database. Click on the link to access the database of all entities authorized to do business in the state.
Search the database for the name of the company. The database will contain the company's official registration with the state and will reference the company by its official name. The state registration in the database will explicitly state whether the company is a corporation or a limited partnership, because each type of entity is formed under a different state law. Using the state database to distinguish between corporations and LPs is also useful if a business has changed its name or entity type, since the complete history of the company will be obvious in the state record. If the entity database is not available over the Internet, you can call the secretary of state's office directly to research the public registration of any business operating in the state.
Go to the website of the National Association of Publicly Traded Partnerships, if all else fails or if you want to verify that an LP is actually registered with a US stock exchange and selling shares to the public. Access the organization's list of PTPs that are currently traded on U.S. exchanges. Check to see if the company is on this list.