For content writing specifically, I find that I can screen people pretty well before I hire them by asking for writing samples. I haven't been able to do that with programmers because I find it harder to judge ability based on a source file or even repo I know nothing about. But if a writer sends me an article that they think represents their ability well, that tells me most of what I need to know.
Other things I look for in hiring:
* Can they communicate well in writing? If I'm outsourcing something, the best possible outcome is that I find someone who I can just delegate tasks to and they deliver. If they're asking questions that are unclear or that indicate that they haven't read the instructions, those types of miscommunications will likely continue if I hire them. * Did they take a genuine interest in my project or are they just shotgunning out the same application to everyone? Some of my favorite hires are people that spent 30-90 minutes on their application, writing a cover letter saying what interests them specifically about my project. It's not a perfect predictor of success, but it's usually a very good sign.
As the client, I try to specify the job up front as clearly. Good freelancers don't want to work with clients with only a vague sense of what I want. I do it in a public Google doc. Any time the freelancer deviates from what I had in mind, I revise the spec to clarify the point of confusion. That way, the freelancer and I both can see it, and if I drop one person and hire someone else, I don't have to re-explain everything.
I also wrote a couple blog posts about what I've learned about hiring cartoonists and editors:
* https://mtlynch.io/editor/ * https://mtlynch.io/how-to-hire-a-cartoonist/