It's often harder to tell exactly when you're being scammed by a person over the phone or face to face, so hopefully someone can give better details on that regard, but here are a few ways to tell if you're being scammed through an email:
Most businesses will have their own email domain.(eg Tom's hardware may be like firstname.lastname@example.org). someone emailing you business related materials from a yahoo/hotmail/gmail account isn't professional. Most of the free online accounts are for personal use.
make sure that the email is from the country it claims to originate. for example, South Africa is .co.za (returning to our example: email@example.com is clearly from SA). Some companies will use .com (US), but these will normally be big companies with global reach. So as an example, if the person says they're from the UK but is using a .com.hk (Hong Kong) account, be wary.
Typing it into google with the word "scam" after it will bring up any possible information on whether it is a scam or not. Even if it isn't a scam, you can read people's remarks about the particular email, which can be valuable (eg, if a company sends an email promising great customer service and the google search brings up a lot of people saying "service was awful" "what a lie" etc)
Look particularly at grammar. A company shouldn't be sending out emails with bad grammar (eg I woul like to inform you, your a winner in our europiean Loterry).
If it sounds too good to be true, often it is.
Be very wary of anything that requires you to disclose personal information, banking details, credit card numbers or give money.
If they say something bad will happen to you if you don't take up the offer (if you don't reply/ send this in 48 hours, you will have bad luck for 7 years)
IF YOU'RE NOT SURE:
There will be some brilliant scammers out there who will take you for a ride and you'll only know as everything falls off the tracks. If you're suspicious, or just want to check the validity, you can always copy the subject and paste it into google with the word "scam" after it.
Alternatively, if you think it is a scam, or KNOW it is a scam, why not post it on the forum ^^