What state are you in? Most states have that info online nowadays. To get a judgement against you, they would have had to "take you to court" and you would have to been served. However, if the address that was given to them was no longer valid, they would have to serve you by publication, at least it works that way in Arizona, give or take a few steps... If you can prove nothing changed in your address and they didn't make an effort to have you served, you can fight it and have it thrown out, but it's not going to be easy, but worth a shot if you can prove it. I had a collection agency for my ex-fiance report a 5 year old collection, claiming they sent a letter to the address they had on that file, which was a different State, but when I pulled his credit (which is how I found the collection) they had done an inquiry (which is VERY common for collection agencies to determine if it's worth pursuing) and I called them out on it pointing out they knew his current address and should have sent the letter to it, but was being sneaky! They removed it from his credit report and then he settled it with them.
If you are a subcontractor, do you file a 1099 for taxes? If so, then they can't attach your wages from the contractor you work for, technically you are self employed. Again, that is how it works in Arizona. What state are you in and I can do some research or give you some links to look through... Court in general is public information, so anyone with a name can look that info up.