To levy or file liens on judgment debtor's income and assets located in WI, you will have to domesticate the CA judgment in WI. This is called a "sister-state judgment." You will need to know WI law for the process of domestication. You might be able to domesticate it yourself, or you might need to hire a WI attorney to domesticate it for you.
Tax returns are constitutionally protected as being private. However, there are other ways to determine assets and income. You can either send written questions (read The Rutter Group "Enforcing Judgments and Debts") or hire a WI attorney to conduct a judgment debtor exam in WI.
You may obtain the information about where the debtor works by conducting a "debtor examination" in which you may require the debtor to answer questions such as where do you work, where do you have your bank account, and ask for information on other things that the debtor owns.
If your debtor is out of state, you will need to apply to the courts where the debtor lives for help to enforce your judgment, unless your debtor's employer or bank accounts are located where your judgment was granted.
Hope this perspective helps!
Regarding collecting in WI, few things
(1) You will need to domesticate the judgment to WI and enforce it under WI law. The CA courts do not have jurisdiction to enforce against out of state people/entities, unless those entities have assets in CA that can be enforced against (because CA courts would have jurisdiction then)- Domestication process can be easy sometimes and not that easy sometimes. You need to check into laws in WI, or else hire WI lawyer to do it, if you do not want to go to the trouble.
(2) Re finding where the guy works, I have HEARD - now mind you this is unverified, but I have HEARD that state tax returns are NOT privileged in states besides CA. (They definitely are in CA) Therefore, you may be able to subpoena them. Again, this is not verified.
In a practical sense, you may be best off hiring a PI to do some good old fashioned investigative work (aka following the guy to work in the morning, is how it's usually done). This could conceivably be done in a not-that-expensive manner. I would probably only do this if your debtor is the type of guy who would have a job, and not a deadbeat...but up to you.
So basically, answer to your Q is that you need to register judgment in WI to enforce, then you can avail yourself of whatever remedies there are over there, including subpoenaing tax returns from him, if indeed those documents are not privileged in that state. But to do that yourself from CA might not be easy. Unless you are a really intrepid legal do-it-yourselfe, you are probably best off hiring an atty unfortunately.
I am an attorney, licensed only in CA. Any opinions stated herein are based on very limited facts, and should not be used as a basis to take actions that may affect anyone's substantive legal rights. My answering this or any question creates no attorney / client relationship.