This is a very good guide, but if you really need to know if the service you want to use, is a scam or not.. you have to visit this site first :
http://www.badbitcoin.org/ They have a comprehensive list of
all reported scam sites. Note, I said ... reported sites... some sites have not been reported, and may be absent from their list. If you get scammed from any service and it's not there.. please report it
to this site to protect other people from the same thing happening to them.