Gold is timeless. It has been used in jewelry for thousands of years and does not lose its luster over time. The very timelessness of gold may make it difficult to determine the age of a gold ring that comes to you second hand. However, there are steps you can take that will help you narrow down the manufacturing date.
Learn all you can about jewelry styles. An excellent resource if you suspect a ring might be more than 100 years old is the "Antique Gold Work" page of the Antique Jewelry Investor website (see References). Sometimes it can be hard to know the difference between a truly old piece and a newer piece made in the same style, so it's helpful to remember that older rings are, of course, likely to exhibit more wear.
Look for marks in or on the ring. Because old gold often shows signs of wear, marks may be hard to see. Use a jeweler's loupe, a small magnification device used to examine jewelry.
Use marks references to learn all you can about the marks you find on the ring. Marks references are simply books or other reference guides that offer explanations and examples of trademarks, quality or quantity stamps and area-of-origin identifiers. The National Stamping Act of 1906 requires all jewelry made in the United States to be stamped with the maker's trademark if it is also stamped with a gold-quantity hallmark. This means if a ring is stamped "14K" and has no other marks, it was either made before 1906 or outside the U.S. Likewise, if a ring is stamped "18KP," it was likely made after 1981. An amendment to the act in 1981 tightened purity guidelines, and many manufacturers began adding "P" to their stamps to denote "Plumb." Clues like this add up and can eventually help you determine the age of a ring.