The short answer: If it looks new, it probably is.
For example, a piece represented as Depression glass that doesn’t have signs of wear on the base is probably new. In my experience, even pieces that sat on a kitchen shelf for years without much use still got moved often enough to show a few wear scratches on the base.
And if I were buying myself a collectible Barbie doll from the early 1960s, I'd likely spy a little age discoloration in the vinyl, hair that isn't as shiny as new, and perhaps a little wear to the face paint as well.
But, identifying reproductions overall isn't quite this simple, and there are some other factors that need to be considered before deciding whether something is old or new.
Beware of Being Misled
Unfortunately, there are times when sellers will intentionally deceive you to make a quick buck.
There are tricksters out in the world who will soak linens in tea to make them look aged or beat a piece of furniture with a chain to make it look worn and weathered. There's nothing wrong with buying these items when you know what you're getting, but realizing that not every dealer on the planet is an honest fellow makes sense, too.
Does a dealer have an exceptionally large number of any one particular item? Is something usually hard to find suddenly plentiful? Do you see the same item over and over as you move from booth to booth? Answering these questions positively isn't always an indication that reproductions are present, but these affirmatives certainly send up red flags that should be investigated before making a purchase.
Reproductions are also frequently mass-produced and imported from other countries. From pottery vases to Bakelite bangles, fakes abound out in the flea market fields. A lot of these fakes are obvious to the avid collector, but there are a few exceptions that are surprisingly well-crafted. Many sellers who are new to the business don't know they're serving up reproductions either so education is your best defense against getting taken in these cases.
Arm Yourself with Knowledge
The best thing you can do to guard against reproductions is to learn as much as possible about your collecting interests. Handle as many genuine pieces as possible, ask questions of other collectors and dealers, and join collecting clubs to continue your learning quest.
By studying reference guides, and comparing genuine pieces with those known to be fake, you'll learn to pay attention to details that indicate something isn’t quite right. It's a skill that comes with time, but patience and persistence will pay off.
In the short term, learn to use your instincts to your advantage. If something seems amiss, listen to that nagging voice. Of course, if the piece is bargain priced you might want to purchase it for use as a benchmark. For instance, if you're interested in cobalt blue glass, owning a reproduction piece for comparison purposes isn't a bad idea.
Buy from Reputable Sellers
As you're learning about reproductions, talking directly with sellers is very helpful. A reputable dealer won't mind if you ask questions about how they know a piece is old or authentic. They won't be offended if you express concern about reproductions of similar objects, and they will explain to you how they know their wares are genuine.
Good people in the collecting world don't mind helping one another learn, and they enjoy sharing their knowledge. Sometimes they even like to show off a bit when they're particularly well-versed on a topic. If a dealer isn't communicative, take that as a sign to move on.