If you think someone is using your trademark, you will want to seek a trademark attorney, but before you do, you should make sure a trademark infringement is actually occurring. Remember, infringement of a trademark happens when someone else uses the same trademark for competing goods and services, and customers are likely to confuse their business with yours.
Also, unless your trademark is well-known across the country, location will be important to look at. If someone uses your trademark for a similar business or product two states over, it is unlikely to confuse customers. Also make sure they are using an identical name or mark.
If you are convinced that your trademark is being infringed upon, you should definitely take action. Your first step is to contact a trademark attorney. He or she will probably start by sending a cease-and-desist letter demanding that the other entity stop using your mark. If they do not stop, your attorney will file a lawsuit in federal or state court. If usage of the mark spans more than one state, they will file in federal court; if usage is local, they will file in state court.
Remedying the situation includes stopping any further use of the mark. In addiion, you can ask that the infringer be made to pay you any money he or she made while using your mark, or compensate you for any damages you have experienced as a result.
Trademark laws can be complex and confusing, but they are necessary. If you do not file a trademark, you can still argue rights to your business name or mark by "common law." However, it is much more difficult to prove infringement if you have not filed for federal trademark protection with the USPTO.
Source: Entrepreneur, "What to Do If Someone Uses Your Trademark," Nellie Akalp, accessed Feb. 16, 2018