How to check originals?

I just got to know from a friend that there are ways to check if a product is an original or not, specifically when purchasing branded stuff. We were buying a well known brand of shoes at one of the biggest malls-he intently checked and instantly knew if it was the original; while there were other pairs under the same brand name being sold as originals though they were not. Is there a website that lists these? How can I know if the shoes (Nike, LC, Adidas,etc.); handbags, clothes, watches, perfumes, etc are not duplicates? I don't want to waste my money on fake, cheap immitations nor do I want to be duped.