You use their gross income. That would be found in Box1 Form W-2 or Box 7, 1099-MIsc.

@Russ_aye

My student son graduated mid-year and got a good job. He is now living in my home. I've paid virtually all of his support for the year, so his savings have increased by an amount nearly equal to his take-home pay for the year. That bodes well for the support test. But he paid a lot of income tax, and if I must include that (i.e. use the W-2 box 1 number for his income, then the support test will count his taxes as self-support expense, even though he never had use of the tax money that was withheld. That seems unfair in a way. So, I need to stop complaining and use W2 box 1?

The IRS has a worksheet in determining support, as found in the link below. The first entry includes the income of the person you supported. The description indicates that this is taxable income, so yes, you must use box 1 wages from the W-2. You cannot use after tax income in determining support.

IRS Worksheet for Determining Support

Rules for Claiming a Dependent