That rule is pretty much dead now, so you can stop looking for the checker tool. Facebook's system basically makes a judgment automatically. If it thinks there's too much text, it might just reduce your reach a bit or you'll see higher costs. It's not a hard pass/fail thing anymore, and honestly, I've seen ads with loads of text do just fine.
The bigger point is that you're probably asking the wrong question. It's not about hitting some arbitrary text percentage; it's about whether the creative is effective. An image cluttered with text is usually just a bad ad and won't perform well anyway, regardless of any rule. The goal is to grab attention and get a single message across instantly.
The only "checker" that matters now is split testing. For instance, in a campaign for a women's apparel brand, we tested a beautiful lifestyle image with minimal text against a version that explicitly called out a special offer in the creative. We ran them in the same campaign, and the data told us which one was better. That's how you actually optimise for performance, not by trying to follow some outdated rule.
Hope this helps!
