What if it is already a judgment, but the person has skipped out on paying it. How do I go about ensuring that I'm not wasting more money by serving the wrong company he worked for, if he no longer works there.

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

There are several ways to check whether someone is employed by a company. The easiest way is to just call and ask for him. Generally, they will tell you he is not longer employed or connect you.