Let's face it - vintage is cool. That's why all the department store are bringing out "vintage" lines. So dating simply by style is about the best way to make a mistake. Plus the thing about fashion is that every era is drawing on what's come before - there's a great ad in my 30s book:
You obviously need to know what was in fashion when, and this is a huge subject - and I'm not going to write a book about it, but rather point the way to resources that will help.
The best books I ever bought were these:
They're about $10 each - and if you get them from Amazon, you'll get free shipping on orders over $25. Believe me, they are worth their weight in gold as it goes through the decades year by year, and you begin to get a feel of flow - which makes it much easier to spot a treasure.
Also - check your thrift stores and estate sales for old fashion magazine, you'll begin to see trends in construction and material that will make it easier for you to find the real thing.
This is just an overview - I will later go into each decade in detail - but for now, check out this link:
Costume History at the Costumers Manifesto
It's a great collection of links decade by decade with photographs and articles for you to look at while you're waiting for your catalogs to arrive.
In the next installment - Know your materials - what was used when and how.