How can I find my code?

I recently bought a new house and the previous owner did not give me the code for the alarm system. It was monitored by ADT and he said they never used it and don't remember it. ADT is not monitoring it anymore so they won't give it to me. I do not want to go with any monitoring company but just want to use it as local alarm. How do I figure out what the code could be and reset to what I want......