40 ILCS 5/1-104.2

Beginning January 1, 1986, children not conceived in lawful wedlock shall be entitled to the same benefits as other children, and no child's or survivor's benefit shall be disallowed because of the fact that the child was born out of wedlock; however, in cases where the father is the employee parent, paternity must first be established. Paternity may be established by any one of the following means: (1) acknowledgment by the father, or (2) adjudication before or after the death of the father, or (3) any other means acceptable to the board of trustees of the pension fund or retirement system.