While there is no way to guarantee that you are dealing with a legit site, there are "red flags" to look for in a fake sites.
1) Payment options, do they include Western Union, moneygram, paypal and bank transfer? Some scam sites will accept credit cards but most prefer those 4 options which are anonymous for the scammer to pick up the cash and disappear.
Western Union and moneygram do not verify anything on the form the sender fills out, not the name, not the street address, not the country, not even the gender of the receiver, it all means absolutely nothing. The clerk will not bother to check ID and will simply hand off your cash to whomever walks in the door with the MTCN# and question/answer. Neither company will tell the sender who picked up the cash, at what store location or even in what country your money walked out the door. Neither company has any kind of refund policy, money sent is money gone forever.
Paypal can only get your money back if there is money left in the scammer's paypal-linked bank account. Scammers know this and will immediately withdrawal your money and disappear. No money in the scammer's paypal-linked bank account means absolutely no possibility of refund for you.
Your bank can only get your money back if there is money left in the scammer's bank account. Scammers know this and will immediately withdrawal the money you transferred. No money in the account means absolutely no possibility of you getting your hard-earned money back.
2) Contact options, is it a free email address such as gmail, hotmail or yahoo? Is it a chat box? Scam sites will rarely list a phone number or street address.
Scammers love to create free email addresses and rarely will use a paid server. Email is easy to ignore and block and free email addresses are easy to open and close completely anonymously. Chat requests are easy to block via ip address.
3) Shipping options, do you actually get to choose the option at check out? Fake sites will frequently say "free shipping" and "tracking numbers emailed" but, if they ship anything at all, will use the post office, cheap, slow and no account number needed.
Fake sites will frequently show icons for UPS, FedEx, DHL and TNT but then at check out will "ship" via EMS, the Chinese post office. If they send a "tracking number" good luck getting it to work on the EMS website. You will need even more luck trying to contact EMS when your tracking stops and your "package" is lost somewhere.
4) The icons at the bottom of the page, are they just copy/pasted pictures or links to actual sites?
Fake sites will often have icons for Verisign, McAfee, Paypal and other companies at the bottom of the home page. Those should be live links to that company's website. Fake sites can't risk linking to real sites so they just use badly copied pictures instead.
Whenever suspicious or just plain curious, google everything, website addresses, names used, companies mentioned, phone numbers given, all email addresses, even partial sentences from the emails as you might be unpleasantly surprised at what you find already posted online. You can also post/ask here and every scam-warner-anti-fraud-busting site you can find before taking a chance and losing money to a scammer.
There are scam busting sites with online lists of the names scammers use, their email addresses, stock copy/paste emails, paid-for-in-cash cell phone numbers, stolen pictures and fake websites they use. You could start your search at one of those sites.