Content writer here

So I write SEO-based articles part-time, and have been making a living from doing this for about a year now. The thing is that I'm not so good at the technical stuff when it comes to users' websites.

When people ask me to take a look at their new site, I tell them that they'll have better luck talking to a dev as I'm just a writer. I'm unable to tell them if the site is easy to navigate, or stuff about site loading times, autoresponders, etc.

Being an SEO content writer, I guess I should know about this stuff as well. I do plan on working full time when college ends. So, I'd like to know, how does one do an audit of what the site needs and what it doesnt?