These days it can be very difficult to tell if a site is trustworthy or not. Many nefarious sites are being designed to look respectable. Thus you should always make sure that a site is not dangerous by using multiple approaches. This is especially important to consider before providing a site with sensitive information such as credit card numbers, banking information, your email address, etc.
In general you may want to be wary of a site if it asks you for unnecessary personal information, a credit card number, or a bank number when it's not necessary. This could be evidence of them phishing for your sensitive information. You should also be wary of sites with offers that seem too good to be true, have very intrusive ads, have multiple popups, tell you that you need to install a plugin to view content, etc... For sites such as these you should definitely consider using the methods described below to make sure that the site is actually safe before proceeding further.