Illinois is not a common law marriage state, according to Unmarried Equality. There are only 17 states that recognize some form of common law marriage, and even where it is recognized, it may be limited to those common law marriages that were created before a certain date.
Common law marriage requirements vary by state. Generally, the couple must at least hold themselves out as being married to the public to be considered common law married. If the couple is common law married in one state where it is valid, then the marriage will likely be considered valid in another state even if the second state does not have common law marriage, as Unmarried Equality explains.