There are no obvious traits to tell who is a scammer or not but if they are repeat scam artists a simple google search can reveal who they are and can be avoided.
Sometimes a good service can go south and turn into scam without obvious signs,just like big firms turn into bad debtors..
Some people are smart enough to use a different handle though. If you can't find anything on those guys(including the people you are trading with), then just be careful and use escrow if you think you still can't trust them.