
A "site free from scam" ? Hmmm...
I am not sure it will be easy to spot one with all of the scam companies and writers floating about.
You won't really know whether or not an academic outsourcing company, or a writer is on the level or not until you have tried to use the service.
If you have a successful transaction, then you found a legitimate company / writer. If all hell breaks lose, then you ended up being scammed. You will have to do your homework though and investigate the companies you are considering for the work you need to have done.
A Google search for the company url usually turns up some interesting leads and ratings which can help you decide about the legitimacy of a company.
As for the writer, you need to contact him directly, speak to the writer personally (whenever possible) so that you can ask the right questions and decide if the accent you are hearing is legitimately American / UK / or Australian. Then you can decide whether or not to use the services of that person. Ask for a referral from a friend who has already successfully used a service instead.
Normally, these referrals are qualified and results in contacting a good company and writer in the end.