How do you know which websites are safe

We all know that there are some web sites that it may not be safe to visit. And there are sites you should not download software from. Some sites and downloads are used to spread viruses or spy-ware but how do you know which sites are genuine and which sites are best avoided. McAfee SiteAdvisor is the 5-star rated Web safety tool that helps keep you safe from adware, spam and online scams as you search and browse the Web. It gives you a symbol next to each link to advise you whether it is likely to be a safe site or not.