The best way to determine whether a company is incorporated is to check with the Secretary of State in the state where the company is incorporated. You can usually search the websites of each Secretary of State by the corporation's name. In addition, corporations must usually register in all states where they do business, so check with the Secretary of State for each of those states as well.
You don't have to jump through any hoops or even pick up the phone to find out if a company is incorporated in a state. Different states have slightly different ways for searching their online databases of companies registered with the Secretary of State. For example, the Kansas Secretary of State's website lets you search by the business name, the entity ID number, by keywords or by the registered agent's name. Missouri's Secretary of State's website just allows you to search by business name, registered agent or charter number. Regardless of the state or type of search criteria, if the company is incorporated there it should show up in the search.