The court can require documentary evidence, such as pay statements, profit/loss statements of sole proprietorships, and tax returns, to be produced and certified as true under penalty of perjury. The intent is that all income received by a parent will be considered when his/her net income is being calculated.
If you suspect that a party is hiding income -- such as by not reporting "cash" or "off the books" income -- it may be difficult to prove, but an experienced lawyer usually is able to help you figure out how to do so.