MAN DOING ROOFING AS UNDER THE TABLE WORK

This person works regularly and is usually paid in cash either on a daily 

or weekly basis - an under the table earner or working off the books.

 Ask your  attorney or the government child support agency to do a credit check to find assets.
If non-paying parent is purchasing a home, car, boat, etc., subpoena the credit application from the lender. Income will be listed on the application. If property is found, you can have a lien placed on it. When the property is sold, you will then have claim to part of the proceeds earned on the sale of the property.
Get pictures of the non-paying parent at work to show the court that he/she is truly working.
Gather evidence. For example, if the non-paying parent installs carpeting for cash, have a friend contact the non-paying parent and ask about prices, and obtain references from someone he/she has installed carpeting for in the past. These customers can then be subpoenaed or deposed to gather evidence needed to show the court that the non-paying parent has income and chose not to use it to support their children.
Once you have gathered evidence to show that the no-npaying parent really does have income you can file a Contempt of Court motion with the court or possibly Criminal Nonsupport charges.
If you are unable to gather evidence to prove that the non-paying parent has income ask the court to issue a seek-work order.