Create a "small copy-test".

Example Scenario:
"Our company is creating a new home page. Our goal is to optimize conversions. Write 'above-the-fold' copy you believe will optimize 'Start Trial' button click-throughs. Please complete within 96 hours."

Give this "small copy-test" to anyone you're considering to hire. Some agencies/freelancers/people will be offended Forget them. The people that know copy 'better than you' will not mind proving it if you have a good opportunity.

The key to a great copy-test is to keep it small and relevant.

Good luck!