70 ILCS 5/2.4

In determining the population of the territory, or of any included municipality, the last federal, State or municipal census for the territory or municipality shall be used, and the population of that part of the territory for which no separate census previously had been taken may be determined by a census taken under the direction and supervision of the circuit court at any time prior to the entry of the order determining and declaring the result of the election, or may be established by other competent evidence.