Turn over your cookware so the bottom is facing up. Make note of all markings on the bottom of the pan and their location. At various times the Wagner Manufacturing Company used curved and straight logos, with or without the "Sidney, O" designation underneath. Some older pans have no logo at all. Pans may or may not say Made in the USA. Also look for a size designation, usually a number, such as 8.
Examine the construction of the pan. Very old pans have a raised ring around the bottom. This kept the cookware from direct contact with the top of a wood stove. Study how the handle is connected to the cookware. Note if there is a sharp ridge or a hollowed-out section where handles connect to the bodies of skillets. All these things provide clues to the age of your cookware.
Consult a reputable guidebook. "The Book of Griswold and Wagner" is favored by many collectors. Compare the pictures in the guidebook with your cookware. Compare the markings on your cookware to the markings listed in the guidebook.
Post pictures of your skillet on the forum for Wagner and Griswold collectors. These collectors will help you identify and date your cookware.