When a website is 100% free, you may find that all people joining the site are genuine or are after a long term, meaningful relationship. 

Websites that are free to join, and then charge to contact other members is a good indication that people who are already members are  serious about meeting someone. In this case, you can be assured that if someone contacts you, they are genuinely looking for a partner. 

Website where you pay to join upfront are usually frequented by professionals or sometimes specialist agencies - such are Russian brides or Thai brides.