I went through this. This is the site with all the specific requirements and a link to the Petition you'll file with the registrar's office. I had to provide pay stubs from his job, our tax returns, copy of our driver's licenses, bank statement, lease agreement, and marriage certifcate, etc. I included anything I could to help since I had it. Lots of paperwork, but I didn't have an issue beyond them requesting more paperwork, haha.
Residency Status Requirements - UAPS
It states: "He/she must rely upon gainful employment in Illinois or prove reliance upon resources in Illinois for more than fifty percent of the income sufficient to provide for tuition, fees, and normal living expenses, e.g., food, clothing, housing, and transportation. Income earned as a result of University enrollment, such as educational loans, graduate assistantships, or student employment, is not considered evidence of intent to establish residency. "