There are a few ways that you can make sure that an online dating site is for real:
1. Ideally, go with a site that has been personally recommedned to you by someone you know and trust.
2. Check out the site's credentials at whois.com and alexa.com
3. Shoot off an email with some questions and see what answers you get/how quickly they get back to you.
4. Are there full, transperent contact details on the site, physical address and landline phone number as well as email?
5. Do you get a free trial period with no questions cancellation? Yes is good, no may not be.
6. Is the site advertising on other high-ranking, trusted sites?
7. Your gut feeling, common sense and intuition.
Finally, I would say just go with one of the big, well-known services such as match.com or eharmony.com which have established themselves as absolutely mainstream. Last I checked, match.com for example was the 70th most trusted website in the world (alexa rank) which says a lot. I have no personal or financial interest in either of these sites, there are others and I offer tham only as examples.