While it's incredibly hard to date many postcards because anyone can print them anywhere and that's precisely how it's always been, there are some factors that can help you determine the era of postcards:
>1898-1919 is known as the "Golden Age of Postcards", when picture postcards were popular
>1901-1906 – undivided backs on picture postcards
>1907-1915 – divided backs on picture postcards1915-1930 – white border postcards were common
>1930-1950 – linen collectible postcards
>Post 1940 postcards were produced as modern chromes, namely color photographs instead of the photochromes generated from black and white photos in earlier postcards – early chromes date from the 1940s to the 1960s.Given that postcards are usually sent within a few years of production, the postmark can be an indicator. Then again, everyone has seen very dusty old postcards that haven't sold for years in some stores, and there is nothing to stop anyone from posting a very old postcard 50 to 100 years later if they feel like it, so the postmark isn't always a good indicator!
>If the postcard presents a city, street or other view that can be matched to photos of a certain time, that can help to date the postcard.
Once you start getting quite serious about collecting postcards, you'll learn that there are quite a few things to take into account when collecting them. As a beginner aiming to have fun with your hobby, simply be aware of these initially and over time, you may want to direct your attention more to the collectible indicators to ensure that you have a postcard collection of the highest quality should you wish to show it at a special event or sell it for good value.
Things of importance when collecting any paper collectible such as postcards include condition, age, anything interesting about the postcard such as its age, the artist/photographer, the image, sometimes the printer, and the postcard's rarity. Other things that might be notable include the author of the writing on it, the content of the writing, the address, the stamp/postmark, and any other elements such as the post office marking the postcard as "prohibited/censored" due to censorship or war, etc.
Any one of these elements alone could cause a particular postcard to be very special and any of these elements could also form the basis of your collection. For example, you might want to collect postcards by famous people or by women from a certain era or that have never been posted and are in perfect condition. In terms of condition, postcards are rated as follows (the first three ratings being for serious vintage postcard collectors:
>Mint: This means that the vintage postcard appears as it would have fresh off the printing press. There cannot be any writing, postage marks, creases, bends, etc. on the postcard for it to be in mint condition. They should be stored in acid-free, archival covers and kept safe from being bent.
>Near mint: This is almost like mint except for a very minor flaw such as a little yellowing at the edge. There cannot be much of a flaw though or it starts to fall down the list. Again, this should be stored in acid-free, archival covers to protect it for the long term.
>Excellent: This is a vintage postcard that is in excellent condition, so no tears or wear. It can be postmarked or written on, provided the postcard itself is still in perfect shape. As above, keep this in acid-free, archival covers.
>Very good: This postcard is often mailed, postmarked and written upon but it has very few signs of wear and is definitely one to be proud of in a collection.
>Good: By this stage, the postcard has lived a little and shows signs of its journey. There may be bent corners, a fold, creases, faded colors, etc. This one is unlikely to be worth much unless it's particularly unusual, rare or was written by something famous.
>Fair to poor: The rest of the postcards in their grimy, beaten up, bent, crushed, creased and other states. The reason you keep them? Mostly because they're sentimental, because they complete a set, because you like them anyway, and so forth. Just don't expect to make a fortune from them any day soon!
Taken from an article at http://www.wikihow.com/Collect-Postcards.