To be honest, it's a gamble (forgive the pun). You might want to stick to sites you know of if you're nervous, i.e. your favorite magazines, blogs, etc. Each one should have an official rules page that is very specific about what the prizes are, drawing times and alternative entry options. There may also be a statement about whether the company sells your info to mailing lists. This is a concern for a lot of people so you just have to decide if you want to go ahead and enter if a company does this.
I use a Google Voice number for mail-in entries that ask for a phone number. I don't like giving out my regular number. But Google Voice isn't popular in some circles because of alleged privacy concerns as well. I'll be doing an article on this too.
If any of the info in the official rules statements seems vague or off, I wouldn't trust the contest. Also, like Rosie says, you shouldn't have to pay to enter contests. Hope that info helps a little.