You have asked several questions that are interrelated and the answers are not necessarily easy.
The law provides that the court consider the 1) incomes of both parents; 2) the number children to be supported and 3) the amount of custodial time each parent has with each child. Being married or single only impacts the tax bracket you fall under when considering the amount of net disposable income each parent has.
The law that controls can be found in the California Family Code section 4055, which is a formula known as “guideline support”. “DissoMaster” is one software program that enables attorneys and courts to quickly calculate the amount under the formula. California has it’s official calculator which also applies the formula. (You can go to our GELLER CONRAD, LLP website and link directly to it on the Resources page.)
If your ex is working part time and not making any money, you will have to convince the court that he is capable of earning substantially more money and has the ability and the opportunity, but is voluntarily underemployed at the expense of the children. This may not be able to do. The court could impute a greater income to him, (calculating the amount he should be earning when applying the guideline formula) thereby lowering the amount you pay.
It is not the court’s responsibility to audit your ex’s finances. This is your responsibility. You can hire a forensic accountant to do so. If your ex does not cooperate, then you can ask the court to order such an order seeking to determine his real controllable cash in his business. If you are correct and demonstrate that your ex is hiding money, the court will modify the support order accordingly and the court may also order your ex to pay for the forensic expert. The real problem you will face is the cost of proving that your ex is earning substantially more than he is reporting to the court.
Disclaimer: I am licensed to practice law in the state of California. Therefore if your case in not in California, the information contained herein may not apply. This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.