Instructions
Look at the brand name on the bottle. Pepsi's true name was Pepsi-Cola in 1903 and antique bottles will still bear the Pepsi-Cola name up until 1953.
Check out the label. Reproduction bottles will have a paper label, where antique bottles simply have the label blown into the glass.
Look through it if it is a brown Pepsi bottle. A true antique Pepsi bottle is made of glass so dark that you couldn't read a piece of paper through it if you held it behind the glass. Reproduction bottles are much lighter.
Measure the neck of the bottle. Real antique Pepsi bottles have smaller necks that those that made today. A simple side by side comparison should tell you whether or not it is a true antique.
Note whether or not the bottle says "Diet" on it. Diet Pepsi was not marketed until 1983, so if your bottle says "Diet" on it, it is clearly not an antique.