In some states, contractors must be licensed, while in other states, they only need to be registered. While similar, they mean different things. Licensing requires contractors to pass exams and meet certain criteria in order to prove competency. Registration does not require the worker to meet criteria or pass exams; it is a written record of the person performing the work.
If your contractor is licensed, their license number should be listed on their website, social media, or any type of quote sheet or contractor. If you don't see it, ask for it; your contractor's license number should be readily available. If it isn't, be wary.
Learn more about contractor licensing requirements by state.