1. Strategic ability - Do they have some strategic thought behind their creative processes or are they being creative, purely to be creative?
2. Do you find that their work embodies "salesmanship in print" or are they just creative writers? Some people can write very creatively but it does not mean that they know how to write good copy.
3. Are they able to write in different styles so they can change their writing to reflect a particular industry group? Are they able to adapt to writing for different organizations?
4. Can the copywriter visualise and communicate a concept to the designer? The ability to visualise a product/concept and communicate this in writing is a very useful skill and one that is important for a good copywriter.
5. Market research - How do they propose to "get to know" your market? By guesswork, or will this be based on their experience? Will they roll up their sleeves and interview the clients?