There are certain traits that often give away fraud. Here are a few things to watch out for that may indicate you are dealing with a sweepstakes scam:
Since many of these scams come from individuals for whom English is a 2nd language, they will often be filled with glaring errors in spelling, grammar, and punctuation. Even if the message is typo free, look for strange wording as if it was poorly translated.
It's possible that some smaller, legitimate sweepstakes sponsors could notify you with a free email address, but keep your guard up if the email does not match the company from which it supposedly originates. And especially if it comes from a random address like "email@example.com".
As far as postal mail goes; legitimate sweepstakes administrators rarely send out win notifications by bulk mail.
They use first class postage with delivery confirmation, or services such as Fed Ex to deliver notifications.
You are not addressed by name
False winning notifications are sent out by the hundreds of thousands, especially when sent by email. Often, they are sent without knowing the name of the people they're contacting. If your win notice has a generic salutation like "Dear Sir" it's a good indication that it's fraud.
You are asked to send cash up front
If you are required to pay money for ANY reason in order for your prize to be delivered, you are likely dealing with a scam, or an illegal
sweepstakes. The exception is paying port, or air taxes on a travel prize.