Pay attention to an online merchant’s website. If they offer too good to be true deals, they probably are. Be careful when buying products from them.
Check their ratings and/or reviews, including other products. If they do not look trustworthy, be careful when using card credentials with them.
Register an account with them before buying a product. You would be able to see something non-standard in an account opening, that would lead you to spotting the low security making business with them.