How do you know an email is an online scam?

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I'm not really a big fan of internet but I enjoy browsing my lot and some other social networking sites. However, I noticed that a friends email look suspicious so I called up my friend and confirmed that the email add was indeed hers but she sent no such kind of email. Now, this is frightening right? I mean considering the implications..it means your email account can be hacked right? How do you recognize potential online scams? I always get these ads on my email and I just delete it automatically out of suspicion that it might be bogous.