Understand what a fictitious name or trade name is.
A fictitious name or trade name is a name used for business purpose. Obviously, a business named " Best Food Store" and owned by a person uses a name other than the name of the owner. But, corporations, partnerships, limited liability companies, and the like are considered to be persons, also. We know when we go into a grocery store name "Publix" that it is owned by Publix. But, Publix might be owner of the "Best Food Store" also.
Understand why it is important
The importance of properly identifying the owner can not be emphasized enough. The real goal of a law suit is to get paid. Often, defendants will not contest the case, and a default judgment will be entered. How will the plaintiff get paid?
The laws of each state give judgment creditors the right to use certain legal means to take property belonging to the judgment debtor. Typically, those means include levy, which is the sheriff taking an asset, such as a car, equipment, inventory, and garnishment, which is having a writ served on someone or something that owes money to the judgment debtor, such as a bank or employer.
Those types of assets require the name of the owner of the business, not the fictitious name they use.
So, a judgment against "Best Food Store" will not find assets of the debtor subject to enforcement of the judgment.
Obtaining ownership information--Fictitious name registration
Most states have laws which require a fictitious name and the address at which the business is located to be registered with some state-wide government agency, typically, the office of the state Secretary of State, and to provide the name and address of the owner utilizing the name. Fictitious name registrations have a limited life term--in Florida it is 5 years--but can be renewed. With on-line computer access, the search can be completed in minutes, but those without computer access can write to their state's Secretary of State, or similar entity.
Local governments typically require businesses to get an occupational license, or pay an occupation tax, or business tax, or similarly named fee. Whatever it is called, it is a way that local governments obtain revenue. Check the tax collector for the county in which the business was located, and also, the city, township, village or other unit. A business operating in a city, town, etc. must get a license from both the county and the city.
Obtaining ownership information--Court records
Court files are public records and open to the public for review. A law suit filed by someone else may have information about the owner. One must be careful, though, as there is no guaranty that the plaintiff in that law suit has done their homework properly, but it is a good starting point. And, certainly, a suit by a former business partner who would have access to "inside" information would have more credibility as to the identity of the owner of the business.
Obtaining ownership information--Public records
Public records such as deeds, marriage licenses, mortgages and the like are recorded and open to all to review. Final judgment are also recorded. Reviewing the public records may disclose judgments entered in other counties or state that have been brought your state to be enforced, so even if there isn't a law suit in the country records where the business is located, there may be a court file in another jurisdiction to review.
The existence of judgments having been entered may also be a good clue as to something else--do you really want to spend the time, money and energy to bring a law suit against a business when so many others are chasing the owner for money also, and some of them may already have wiped out a bank account through garnishment, causing the owner of the business to "go underground", which is to say to get good at learning how to hide assets, and making your efforts to collect what is owed much more difficult.
Obtaining ownership information--Ask
Simple enough. Ask an employee working at the counter, or call. An employee may not know that there is a debt issue and be glad to be helpful. You as the caller have nothing to lose by just asking, the worst that can happen is that the phone call may be terminated abruptly.