How to shop vintage

If you go to a vintage store:
1. Know what you like.  Do research on the Internet before so you know if you are more a 1920's or 1940's girl.  Look for clothes in the store that resemble the ones you liked online.  Make sure to ask if the store goes through a vintage certification process.  If it does, keep reading.  If not, treat it like a thrift store and follow the tips for shopping vintage in a thrift store that I have added below.
2.  Try it on and pay attention to the measurements.  Vintage sizes run different then sizes today.  Some stores will include measurements, so know yours before you go in.  Otherwise, just try it on.  Remember that you can hem up, let out, and take in anything you need to with a sewing machine at home.
3.  check for imperfections or tears.  If you find some yet still want to buy the item, ask the sales associate for a discount, normally they will give you one.  Mend any small imperfections at home.
If you are buying at a thrift store or on Etsy.com
1. Follow all the tips above
2. When buying through a store that does not go through a vintage certification process look for these easy details to insure that you are buying real vintage clothing :
The tags:
vintage tags look much different than ours do today.   Here is what to look for-

(on the left) size tags (this one from the 1940's) are normally a small loop of fabric.  Look for a number, rather then size letter (XS, S, M, L)

(in the middle) Most vintage tags will no look like the long loop tags that decorate modern clothing.  Up until around the 1950's they were mostly rectangles or squares sometimes with beveled edges.  In the late fifties to early sixties (on right) they began to more commonly stitched to the garment on the sides of the tag.  Vintage tags will normally have a face with just the brand name and where it is made.  These tags normally feel more like fabric then the papery tags we are used to.  The brand names are normally creatively embroidered to match the brand's label and sometimes the stitching is colorful. 

Vintage zippers are traditionally a flat piece of metal, most commonly in a rectangle shape.  They normally have a cutout in the center and it is not unusual to find the brand name etched into them.  If you find an item with a tiny oval zipper which looks like a grain of rice or one where the cutout is really small and close to the bottom, you know it is not a vintage item.