Sometimes being able to differentiate safe e-commerce websites from suspicious ones is by knowing the reputation of the brands from before the internet became so popular. If you typically shopped at brick and mortar stores like Target, Walmart, or Bed Bath & Beyond, you can probably feel safe shopping at their online stores. Another way to tell a legitimate e-commerce website from a suspicious one is to use a search engine to see if there are complaints or comments from other internet users.
Also, you can trust an e-commerce site that uses an encrypted check out system with security protocols such as SSL (Secure Socket Layer) or SET (Secure Electronic Transaction.