First, you should check age of the site. Second, check if this site has a SSL certificate or not and the certificate is free or not. Third, you can create a topic at some forums to get help from other users. Of course, you can get some sites have better service than the site you pay attention to. I think that all you should pay attention to if you want to know a wallet/exchange system is good or not. I think. I hope my think can help you.
P.S: Sorry for my bad english, I'm trying my hard to improve it.