Find someone who knows technology marketing writing, not a manual writer, a programmer or a marketing design firm. Find a technology marketing or creative advertising writer that knows the industry. Go even further, and see if your prospective tech writer can get technical and speak with your developers, engineers and product managers. With the software and hardware industries as broad and developed as they are, try to find a specialist that knows your industry.
Tech writing is a broad category with many different specific disciplines.
The best way is to ask for clips. Determine whether the freelance writer you want to hire is truly professional. Don't waste time on hobbyists that may or not be around when you want them in on conference calls, interviews or meetings. Get a writer with good people skills. Even if they're not going to be working on site, people skills count.