Copywriting, like sales, is all about persuasion. The easiest way to persuade someone is to use language that they understand, and that compels them to do the thing you want them to.

That requires you know who your audience is, and what they want. So a basic understanding of how people respond psychologically to language is useful to every copywriter, but you don't necessarily need academic knowledge to achieve this. You can glean a lot of this from data, and from case studies, from analysing trends within your industry.

In terms of trying to determine whether a copywriter is up to scratch, you need to know your audience, too. You need to understand what they will or won't respond to, so you can provide a clear brief at the outset, and determine if what's provided to you fulfils that brief.