The simple answer is you don’t.
The issue with beta readers that are friends (not friends that are writers, but just your average friend) is they are more likely to be less honest. The goal with a stranger from the internet is to get a cold read on your novel.
You also don’t have to go with the first person that wants to beta read your book. You’re allowed to be picky. Essentially, you can meet them, discuss things with them, and see if they are a person you want to work with. It will be a slower process to find a beta reader this way, but you can establish a relationship. I’d also recommend picking someone known in the community. As in, not from craig’s list. I’d probably recommend Goodreads or Reddit for this one (Reddit with a username that is known - their social media profile, etc)
If you are someone that is overly concerned with your story being stolen, then I’d say a developmental editor might be another good choice.
If you’re really concerned and don’t have money for a dev edit (it’s expensive!), then family and friends are a good last resort, but they do come with a bias. (unless one of them is specifically in the writing community as well and knows how to be objective). Most of mine are terrible beta readers, lol. They just say, “Yeah I like it! Maybe change this, etc.” They are not critics at all, so I can’t rely on them for that.
I’ll add too that this is one of the trade-offs with “having full control” in self-pub. In trad pub, your editor and agent will look at your work. Without them, unless you know a network of writers, you kind of have to start looking on the internet for those resources.