If you're looking at cast iron cooking implements or other items at an antique sale, you can avoid overpaying for that Excelsior skillet if you can tell the time period when it was really manufactured. While unmarked cast iron skillets are just about impossible to date, if you look at the maker's markings, you can at least categorize the pieces by time period and manufacturer.
Look at the city or place where your cast iron piece was made. Between 1865 and 1909, Griswold marked its pieces with the word "Erie," so if you can find that word on a skillet, you can look in a collector's guide and find out how much it is worth at that time, based on the condition.
Read the manufacturer's name, which was generally stamped on the underside of the skillet. You can usually find the company logo, name and main city there. Wagner and Griswold are the two foremost brands for cast iron, but other coveted manufacturers include Excelsior, Dixie, Sidney, Favorite, Columbus and Piqua.
Check out the surface. If the surface has a matte-based sheen to it, it may have nickel plating, so it's definitely after the 1890's, when that process was invented. During the 1920's and 1930's, skillets were inlaid with porcelain on the top and bottom.
Analyze the heat ring that goes around the bottom side of the skillet. If it's closer to the outside of the skillet's circumference, it was likely made before 1905, when these heat rings moved to the center of the skillet.
Check out the handle. If it's wooden, it was most likely made between 1885 and 1900. These handles were popular because they didn't conduct as much heat and were easier to pick up, but they also would crack because of the heat and dryness involved in the cooking process.