The very nature of the job requires that tech writers understand what they are writing about, without the advantage of a Ph.D. in mathematics, astrophysics, software engineering, or whatever subject the tech writing is writing about. A technical writer must also be a good investigator. It is not uncommon to have to dig up information not clearly listed in a design document. Finally, a tech writer must be able to quickly learn new authoring tools. The media on which writers deliver documents has changed dramatically with the growth of the Internet, and a writer must be able to keep up with the latest in authoring tools and technology.
Most tech writers can master new software publishing tools relatively quickly.
On the other hand, learning to effectively use the English language so that your audience will receive and understand your communication takes much longer to learn and master. So focus on a writer's ability, and not the latest buzz words. If you're only looking for contract writers to help for a few months, then obviously you would want them to have the skills you need before you hire them. There is no magic formula to see if a writer writes well, other than, of course, reading something he or she has written. Art is about communication , and if someone has a desire or passion to communicate as an artist, then it's likely that his technical writing has a quality that will ensure it communicates as well.