Hidden Income

[TwistingTheKnife]

My ex is self employed in a cash business and lies about income.  Ex is now filing for support of my 18 year old child (who is still in HS). How to I protect myself from being raked over the coals when the assets are hidden, making it seem like yearly income is only $15,000 (in actuality its $150,000) compared to my $40,000?