Once my company has posted few topics and asked writers to give sample writing with their bio data and working details. We got some really good working people, If you like you can use this method.
When you're going to hire any writer, you must need to cheek some sample from they guys. When ever you see it's not that what/how you're looking for just stop them.
It was simple to get a qualified writers online. Always look for their protfollo or write ups online. And if you are not satisfied with their write ups. Allow them to write a sample article for you to see if they are qualified.
Get a sample from your content writer and check what he writer in content. I mean check English, grammer, and sentence. if you like then continue otherwise not.