Marriage or birth certificate
A man is considered the legal father of a child if he was married to the mother when the child was born and/or conceived. Or, if the child was born after August 1996, being listed on the child's birth certificate automatically establishes paternity.
Illinois Department of Public Aid determination
Paternity can be established at a Public Aid hearing, where the mother names the father of her child in order to receive aid. Sometimes, men who are named the legal father through this process wish to contest it. This can be done, but there is a time limit.
If the mother does not agree to establishing a father's parental rights, then a lawsuit can be filed asking the court to determine paternity. The required documents are a Complaint to Determine Parentage, as well as a Parentage Summons. These forms must be served on the mother, and sometimes her husband if she is married or was married at the time the child was born. The father can ask the court for a DNA test.
Acknowledgment of parentage form
If you want to establish paternity and the mother agrees, you can both sign a Voluntary Acknowledgment of Paternity form. You can get this form at the Illinois Department of Public Health. If the mother is married to someone else, or was when the child was born, that person needs to sign a Voluntary Denial of Paternity form, also available at the Dept. of Public Health.