How do I report fraud to the IRS?

I know somebody who is suppose to pay back- pay for child support. but has told the child support agency he doesnt work and hasnt worked for a very long time. when i know and have proof that that is a lie. infact, he works for his parents janitorial company and i have worked side by side with him for 10 hrs one day. the way he gets around this is by getting paid for his hrs on a check written by his parents but in his girlfriends name, who also works with him for his parents. from my understanding this is against the law and I want to report this to the IRS to go into investagation. please let me know what my first step should be. thanks.

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