A W-2 employee is obviously more straight forward -- they just use gross pay on W-2s and paystubs. If there is a large variation in employee income, they will take an average over last 3-5 years. Of course, there is an argument to impute a higher level of income to an employee if they volutarily stopped working or reduced their income. And additional income from investments, rental properties, etc. can and will also be considered.
The court has discretion when calculating a self-employed person's income. For self-employed persons, there are specific deductions that the IRS will allow that are not allowed deductions when calculating support. Generally the courts want to see Profit & Loss Statements and tax returns for the self-employed person for the last 3 years. This will reflect income less legitimate costs of business (including taxes and other deductions). They do not typically use gross revenues for self-employed persons.
If the amount of income is difficult or time consuming to ascertain, courts and/or the parties can request an expert to conduct an income analysis and business valuation (stipulated joint expert, dualing experts, or court's own Special Master). The cost is largely dependent on the scope of work and nature of the business to be evaluated.