Apart from having testimonials, you can prove your abilities by publishing content.

You could apply the Pain/Dream/Fix copywriting technique to prove to particular businesses that you understand their expensive problem.

You're probably going to find smaller businesses that can't yet hire full-time marketing or copywriting staff.

Describe their painful problem - describe the symptoms - of not having good, captivating content.

Then paint the dream for them. What does that content do for them to get new leads, or to cement the relationship with existing clients?

And finally, offer your solution, although your own writing will do a lot of the heavy lifting here.

Much marketing seems to try speaking about the service provider's solution: "here are our skills and experience." But I think describing the prospects' symptoms first, you're not leaving them to connect your solution to their pain.

If you'd like to have a call with me to help you save some false starts, I can share more specific advice.