Check for number punched on back of the top button:
The replicas one’s have an off centred 501 punched on the button or they will not have any punch at all. While the real one bears a nice centred punched vintage number on the button.
V-Stitch on front fly:
Look for a nicely stitched V along the front fly, which will be ending near the front button. As shown in figure below, the real one has very smooth V stitched and it is ending near the button but the fake ones have this stitch which is no way where near the button.
Look on the rear patch:
Have a clear look on the rear labelling patch, the fake one will have hard material used for label because the genuine Levis which was coming to market pre 1930 used to have leather labelling. The original vintage Levis will have this labelling made with thin material.
Also look for label on the rear patch because even if the material looks authentic, the labelling should be little off centred, or either it will have spelling mistakes. Most of the fakes also bear 501-0657 sign printed with black ink. The jeans came in 1980s used to have this sign. If it is any jeans other than black ones came in 1980s bearing this label is replica.
Some of the Levis jeans also bear a white patch, which is definitely a fake.