How do you know if a website is a scam?

How do you know if a website is legit or a scam?

• I am looking into buying a pair of beats by dre headphones, and the are many websites that claim to have the most expensiver model for only $144.00. It sounds to good to be true. How do I know if the site is real or a scam? http://www.dr-dre-beatssales.com/monster-beats-by-dr-dre-pro-headphones-2011-new-style-white-p-21.html

• Answer:

  Real Beats are never discounted by more than 20% off retail price That's a Chinese site that's less than a week old and owned by someone who has over 700 other domains also selling knock-offs http://whois.domaintools.com/dr-dre-beatssales.com There is no way they are real