Fake wines are a very real issue, but it is usually a problem in the high end wine market, and not as much of a problem with more moderately or lower priced wines. The reason being that there is a much greater reward for counterfeiting expensive wines. It can be extremely difficult to detect a fake wine from the real thing. Before you can spot a fake, you need to know what the real wine looks like. Obviously, if you have a bottle of Chateau Latour with a screw cap, you know it's fake, but the fakes are never that obvious. If you have Netflix, I would suggest watching the movie "Sour Grapes" which looks into the life of the world's most famous wine counterfeiter.
My suggestion- only buy from reputable sources, but even the most reputable can be fooled.