How can I tell if this is a reproduction?

Whether you're planning on selling or starting your own collection there are some steps you should take to avoid reproductions. I will speak about glass as that's my specialty but the principals apply to any collectible.

Do your research
Books - There are many good books on almost every type of collectible and many are available used or at your public library. All of the glass books I have list known reproductions with descriptions of the differences.

The sugar on the left is a "reissue" of the Depression glass pattern Madrid using new molds. This was sold by Federal and called "Recollection" . When Federal went bankrupt Indiana bought the molds, removed the '76 and produced crystal and pink. The plate on the right is original Madrid and is a lighter and clearer shade of yellow amber. There is also a '76 on some of the Recollection pieces and some differences in the handles in all of the new mold pieces.
Internet Resources- Discussion groups are especially helpful. I belong to two groups where authors of popular glass books are regular contributors and will answer questions. And, trust me, they know about reproductions. There are wonderful sites dedicated to just one area i.e. Candlewick, Carnival Glass or Indiana patterns where the site owners have dedicated hundreds and hundreds of hours and dollars to share their knowledge and passion for their particular collectible. The good ones will have information on reissues and known reproductions. See 'My Favorite Links' in the side bar for links to some sites.
EBay - Don’t use eBay listings as a source of reliable information, but the community board is very helpful. This swan is a modern Asian import. It's a copy of the Vallerysthal covered swan dish and the mark on the bottom is a dead giveaway.
The pink mark, from the swan, is not Heisey although it is somewhat similiar. The clear glass mark, courtesy of ddoty.com is a genuine Heisey mark. The fake shown above is Hsinshu Taiwan for AA Imports. As you can see, the Heisey mark is a narrow diamond while this Asian importers mark is square.

What the heck is this?
If you don’t know what you have, it can be very hard to determine if it’s vintage or not. EBay is a great place to start if you have no idea. Start with a general search such as ‘vintage milk glass bowl grape pattern’ and find a listing with a name for the pattern and then use the name to search the web and try to verify the pattern. Don’t use eBay listings as a reliable source of identification.

There’s no substitute for experience.
Handle as many known authentic items as you can. It doesn’t take long to get a feel for the real thing. Here are some suggestions on where you can examine vintage items without getting your hands slapped.
Antique shows and shops are helpful as long as the dealer knows their stuff. Even if they don’t have specific ids, it’s likely that most of the items in their stores are vintage or they wouldn’t be in business long.
Auctions are fun. You can examine everything during the preview and you can find out what’s authentic by seeing how high the bids go. Just sit on your hands until you know what you’re doing. It’s way too easy to catch auction fever, your hand will jump up in the air all by itself and you’ll have paid too much for who-knows-what. I have to admit that I have done this, and not just once or twice. :P
Thrift stores and garage sales are good places to find vintage items. If you have a good feel for something and it’s cheap buy it as a research piece and then you can always resell it at your garage sale. I’ve learned a lot from my mistakes - and had some big garage sales, lol.
Museums are wonderful. Ohio has tons of glass museums and I’m embarrassed to admit that I haven’t been to any but I’m planning some trips this summer.

Is it in like-new condition?
An item that is in brand-spanking-new perfect condition is probably not vintage. It could be the find of the century, but probably not. Unless you know your stuff or want a research piece you should probably pass.

Is there more than one?
Unless we are talking about something that comes in a set - plates, glasses, candlesticks, where you would expect to find more than one – beware! If there are three identical marmalade servers, or seven candlesticks – same pattern and size, they’re probably a knock-off, especially if they’re in pristine condition. There just aren’t that many vintage collectibles around in mint condition.

What about eBay?
As far as glass goes, reproductions and look-a-likes are getting to be a real problem. Oriental imports and even one American company I know of are making new glass in very similar designs to old patterns. There is new glass that will even pass the black light test. However, there are dealers on eBay who list these items, some of which are very nice, as new. Search for vintage style new.

Another Asian import. This is one of many depression "style" items sold at http://www.denofdecor.com/. They are honest about what they are selling but some of their resellers either don't know any better or intend to deceive. Unmarked.
Use completed listings to see what items sold for a decent price – probably authentic; most collectors know their stuff. Use this as a starting point only; don’t ever count on eBay listings for accurate identification!

Did I mention research?
Ultimately, it’s your responsibility to know what you’re buying, especially if it’s something you might resell. In addition, it’s fun to figure out what you’ve found and learn about beautiful things from the past.

Happy Hunting!