1. Take a really good look at the writer's portfolio. Any writer who's even halfway decent has some work they want to show off. If they don't, there's probably a reason. Walk away, no matter how cheap they are. 

2. While you're looking at those samples, keep a mental count of how many obvious errors you find. Grammatical errors, misspellings, and typos aren't professional. And depending on how picky you want to be, hiring a writer who routinely makes these types of mistakes will cost you, either because it makes you look bad, or because you have to take the time to edit their work or pay a proofreader to do it for you. 

3. Make sure the writer you hire is right for your market. Someone who's only written B2C copy may struggle with B2B work, and vice versa. Few writers are great at both. Ask for specific examples if you're unsure.

4. Read reviews on your writer of choice. These could be website testimonials, social media recommendations (LinkedIn is a favorite of mine for this) or even freelance websites. If deadlines are critical for your business, look for writers whose clients comment on their punctuality. Some of the best copywriters in the country don't make their deadlines, so establish whether this is a priority for you before you hire a pro.

And finally, my best words of wisdom about selecting a copywriter:

Don't read that much into the price. Pricing is incredibly subjective. For instance, I may charge more than a writer in India, but less than one in New York. The difference here isn't quality, but rather cost of living. I may charge more for areas I'm specifically trained in, like Finance, but less in areas where I have less experience. I've seen writers who charged triple what I charge, and their work was completely unusable. So don't assume price is a reflection on the quality of the work. It may just be a reflection on how much the writer needs to pay the bills.