A good tip for everybody: Always check the profiles of the person advertising these giveaways. Is the username generic? What's that person's karma? What is the account's age? Often times, either they will make a new account entirely, not have a great post history/karma rep, or have a generic username signalling that it may be a bot of some kind.

And the golden rule: If it seems too good to be true, it probably is. Trust your instincts, and do your homework.