The first thing to look at is the patina. Is it tarnished? Not necessarily black, but does it look like the surface patina has built up over time? A true old patina will be darker in the crevices and low spots on the medal than it is on the high spots. The words and image on the medal will be lighter than the background because they have been rubbed clean by being touched, while tarnish has been building up in the lower areas without being disturbed.
What is the artistic style of the image? Early 20th century medals have beautiful, delicate flowing lines and remarkable detail. Medals from the 1930's into the 1940's frequently have an art deco touch. 19th century medals might have images that look a little cruder and less refined. 19th century medals are also sometimes thinner and more delicate than later medals.
Is the image crisp and clear or has it been smoothed over time? An old medal that has been worn will usually have the fine detail rubbed smooth from being handled by previous owners. If it's silver plate, the brass colored base metal might show through the worn places. This is not always the case, and there are ways of casting new medals from molds made with vintage medals, but it's usually a good sign to show that a medal is old.
Image on both sides?
Is there an image on both the front of the medal and the back? Newer medals almost always only have an image on the front and a smooth or slightly textured surface on the back. Many (but not all!) vintage medals have images on both sides. If the medal shows a saint who was a martyr, the reverse might show a palm branch. If it shows a saint who was known for purity, there may be a spray of lilies on the back. Often there will be saints who are connected in some way on both sides. St John Vianey usually has St Philomena on the reverse because he had a special devotion to St Philomena during his lifetime. St Francis of Assisi will frequently have St Clare on the reverse because they were good friends, and St Clare started a community based on St Francis's religious order.
What is the shape of the medal? I frequently see oval, base-metal medals of saints being sold as "vintage". And yes, they might be old, but they're not really collectible or considered vintage because they can still be purchased at lots of places today. As my grandmother would say "They're a dime a dozen!"
Vintage medals are frequently circular or oval, but also can have delicate lace-like edges, figural depictions, and other unusual shapes. The rarity of the shape makes it more likely to be a vintage medal.
From whom are you buying?Is it someone who has experience in exonumia? (I'm showing off my vocabulary - exonumia is "numismatic items that that are not coins or paper money." Things like medals, tokens, tags, or badges) (OK, what is a "numismatics"? Numismatics is the study or collection of currency - both coins and paper money.)
What was I saying? Oh, whom are you buying from... If you're buying from the sweet nun who's selling her old, tarnished, and unusual religious medals to raise money for charity, it's a pretty good bet that they're vintage. You know that medal's "provenance" - where it came from and who owned it - it's story. If you're buying from "Crazy Joe's Exonumia" on Ebay - well, I'd take a good look at the photo before I bid. It might be vintage - it might not.
I really don't think many people out there purposely try to pass off new medals for vintage. But I know that there are a lot of people out there who think that they have vintage medals but really don't. It's an honest mistake, and I'm hoping this post will help people know what they really have.
I'm sure that once I post this, I'll think of a couple of other clues that I use to figure out the age of a medal, but this is a good start for any medal collector. Please let me know if you have any questions of suggestions about dating vintage medals. Everyone loves to learn more!