1. Check the whois* at any domain registrar and see who owns the site you are dealing with.  If the domain is protected by a privacy add-on, then question why they need to hide themselves.  If they are using a freebie web site like: something.freesite.com then question why they won't pay the $10 to have a real domain where you can verify who they are.  NOTE: It is illegal to lie about who you are when registering a domain name.  It is against the ICANN rules and the domain can be taken away when reported. 
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2. Verify the telephone number listed on the site.  If you cannot find it in the 411 lookup, then simply call it and verify the person on the other end.  If they have no phone number listed (many do not) then check the whois to see the telephone number listed and verify it that way.
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3. If they use Paypal, check their verified status.  Paypal makes it hard to hide who you are. 
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4. If they have a domain name yet are using yahoo or hotmail email accounts, question why.  All domain names that are hosted have free email accounts that can be used.  Any legit business owner would use their domain email addresses because it's free advertisement for their site every time they send a message.
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5. If you meet a person through a interactive group or message board and ultimately start emailing with them, really listen to what they say.  Watch how they talk.  Even though you do not see this person face to face, you can tell a lot just by listening.  If they are not talking directly to you, watch how they post messages and display themselves.
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6. If you have a funny feeling or doubt the person you are dealing with, then move on.  Your gut feelings about a person should be the ultimate decider.

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Remember: "You can be anyone you want to be - online".  You never know who the person is on the other end unless you verify who they are.  It's your right and your job to protect yourself!
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