How do you dispute a judgement you didn't know about on your credit report.?

I check a credit report and found a judgment on there for a few hundred dollars I never knew about, even filed in a city/state I didn't live in. I knew the guy, he tried to get me to pay for damages on a walkway washing out. My company built the walkway, but he built the drainage with another company which didn't install it correctly. He didn't even live in the city and state it was file. Just happend to check credit report for fraud because of an alert from my bank.