Do you need to know that? What makes them a professional and does it make a difference if it's not their full time job?
A wedding photographer is a specialised job, like any other. You wouldn't want your plumber doing your electrics, would you? As a photographer I always up my skills, learn from every job and enhance my client experience every time. I'm a specialised photographer and this is my one and only full time profession. This makes me a professional.
You need to know this, because it makes your experience with a photographer easier, smoother and faster. I will always try my best to keep you happy and will always learn how to make that experience better as I grow with every couple.
A professional photographer will always have a website with their work to show. They will have testimonies to share from previous clients. They will not start a job without going through the contract as a formal agreement between themselves and their clients. The contract will let their clients know what to expect, ask more questions and cover the awkward concerns if any.
A professional photographer will invoice you for the payment to keep records. They will be insured and registered with the revenue. Just like any legitimate business entity.
All of this makes a difference in the level of professionalism and security when providing a service to you. A full time wedding photographer is certainly more efficient and easy to work with.