You mean whether they are safe to purchase products from with your credit card details etc. Legitament sites should be certified by company like VeriSign who offer SSL Certificates to verify that the websites are safe and protected from internet fraud.
When you visit a website that is protected a padlock should appear next to the url in firefox and on the status bar in IE. If you double click on this padlock it should come up with the information on the encrytion and the electronic certificate supplied by a company like VeriSign. Obviously sites must have to meet certain standards of security before they can obtain a certificate.
There is much more detail than that, but that is basically how sites are validated.
I hope this helps.