I figure that a lot of other people probably ask the same question - how can you be sure that the item you're buying isn't just some 1990 repro piece? Well, there's a few ways..
Brands
Gunne Sax (Later became Jessica or Scott McClintock)
Vanity Fair
Victor Costa
I.Magnin
Yves St Laurent
Christian Dior
Samsonite Luggage (Think old steamer trunks and train cases)
Pierre Cardin
Yves Saint-Laurent
Coco Chanel
Vivienne Westwood
Hubert de Givenchy
It's by no means an exhaustive list, but if you find these guys, you're in good company.
Labels
Even if you find a piece by one of the designers above, most of the time they've been wildly successful (Dior and Chanel are household names today), and are still making clothes. So how do you tell if the label is vintage or not?

- The label is yellowed with age, or faded.
- The label is sewn with thread, and not just printed.
Of course, there are exceptions to any rule. Some brands today use sewn tags for an extra special touch. And some vintage labels are still crisp today. But for the most part, screen printed tags and multi-colored labels didn't come on the scene until the 70s, and most paper products and thread undergo some yellowing over the years.
Construction and Material
Sometimes the construction of a garment is a dead giveaway. Metal zippers and wide seam allowences are things commonly found in pre-70s clothing that have not widely survived to today.
Contemporary clothing usually has serged edges inside, and plastic or covered metal zippers.

Before polyester was a bad word, it was the best thing since sliced bread, and plenty of vintage clothes are made from quality polyester.
Silk, linen, and polyester are all common fabrics found in vintage garments. Aged lace is another tell-tale sign of vintage.
Union Made Tags

This is my favorite thing to find in a vintage garment! The union made, or ILGWU tags are excellent ways to date your piece, and for the most part, dropped out of wide use after the 1980s, so the tags are almost exclusively found in vintage clothing.
The Vintage Fashion Guild has an excellent guide to the different eras of tags, and it's usually the way I date my pieces.










































