Technology writing is not relegated to technical departments. Find someone who knows journalism copy and marketing writing, not a manual writer, a programmer or a marketing design firm. Find a marketing or creative advertising writer that knows the technology 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.