There are only two simple rules to follow:
1. Never give credit card (or other financial) information to a site that you don't know anything about.
2. Never install anything (plugins, toolbars, etc) from a site that you don't know anything about.
If the site doesn't require you to break either of these rules, it's probably legit. If you think the site is worthy of you trusting it, but you aren't sure, do a web search on the site to try to find other people's opinions and reviews. Scam sites that are "popular" will have a bunch of negative feedback.
Oh, and use your brain. Foodnetwork.com is the web site for the foodnetwork television station. Are they going to damage their reputation by trying to scam people that come to their web site? Highly unlikely.