Instructions
Inspect the tags if the garment has them. The first tag you are looking for is a designer label. If you are already knowledgeable about popular designers of past decades, you may be able to immediately recognize the name. If not, you may want to bring along or call a knowledgeable friend who can help you.
Look for the country of origin tag. Again, the garment may not have one as this has only been a more recent manufacturing requirement. However, if you find one that reads China, it is less likely you are dealing with a vintage piece---most were made in France or Italy. Also, care instruction tags didn't begin to appear until around the 1960s.
Try the item on in your size. If it doesn't fit as a standard size, or if the seaming on the sleeves or waist seems higher or lower than contemporary styles, you may have a vintage item.
Check the stitching. Because vintage pieces were more often hand sewn to fit a specific person, the stitching may be less uniform than what you would find with an item that is machine sewn.
Feel the fabric. Most vintage items were made of natural materials such as wool, cotton, or silk, although from the '60s on, you'll find more synthetics like polyester.
Look closely at the zippers and buttons. Typically buttons will be metal instead of plastic, and will be more detailed or intricately engraved. Zippers are also usually metal, brass or Bakelite (which looks like plastic, but is slightly heavier), and are slightly wider than modern versions.