Go to a reputable liquor store and look at the genuine bottles. Take note of the cap seal and label. So at least you know how a genuine one looks like and how you can tell if its fake before buying. Fake ones usually have shoddy label that can be wrongly stuck or the cap seal looks like it has been reworked.

Also look at the alcohol beading. Gently shake the bottle and look at how to bubble rises up to the top of the bottle. If the bubbles / froth are there and it looks like teh tarik bubble then it's very likely to be fake.

These are some tips that liquor purveyor taught me previously.