First and foremost, by from a known quality source/dealer. If buying used from an individual, either know who they are or request the serial number... Leupold will tell you if the scope is authentic or not.
The best thing to do is buy from a legit store or with someone who has a trusted history. I sell leupold and have had to tell people that they bought fake product a few times. It always sucks. The problem is they have started to take the new fakes and sell them as used so as not to have to worry about having the box and what not. Or, even worse, they'll take a new box and put the counterfeit scope in.
If you ever have any questions on whether or not a scope is legit, you can always ask me. Just by looking at pics online, I can't always tell but I might be able to give you a few things to ask the seller about.