Some steps I use to ensure legitimacy:
1. Ask yourself if this is a sweepstakes that you remember entering. If you never entered for the prize that you just "won" the odds of being a scam are probably high.
2. The notification should come from a business or prize service e-mail address. Anything from an e-mail that anyone can create (IE: @hotmail.com, @yahoo.com, @gmail.com, @outlook.com etc. is a red flag.)
3. Correspondence should look professional. Any forms to be filled out and returned should look like they're from a business. IE: Not a bunch of misspellings, bad grammar, etc.
4. If you need to return any forms make sure they are going to a business e-mail address or a company street address/fax # (Just Google the street address or phone # to check)... either of the company putting on the sweeps or the sweepstakes service company administering it. IE: Don't send anything with your SS# to a residential address or a PO Box.
Note: Any prize won with a value of $600 or more WILL require you to submit your Social Security # to the company for tax purposes. This does NOT mean it's a scam. Some companies will also require it for prizes with values under $600.
5. Don't send anyone $ in order to claim a prize. Anything tax-related you will do on your own with federal/state filings.
6. All "foreign lotteries" are scams. Either that or the UK owes me a ton of money. har har.
7. Be careful with what sweeps you are entering. A sweepstakes from Coca-Cola is much safer to enter than from "WinMoneyNOW! LLC" or whatever. Prevention is the best medicine sometimes. Sites like sweepsadvantage are good because 95% of the sweeps they post/screen I feel comfortable with entering. Just avoid anything that gives you bad feelings... always safer that way.
Good luck with your sweeping!
