I believe so. I worked for a number of companies that ran sweepstakes. I guess there are some out there that are spam but if you find ones that are genuine then you've definitely got a chance.
I guess a good way of knowing if it is a scam or not is if you can find more information apart from just a sign up page, for instance being able to see their website or contact them. Also tool for a terms and conditions page which should have information about how it works, how the decision process will take place and when.