Insanely effective tip: watch a junior dev use your docs, offer no assistance, and have them say out loud what they're thinking and doing. You'll discover dozens of ambiguities, 'gotchas', and shortcomings!

It's important to use a new junior each time (i.e. if they've used your docs before, don't use that person again). They should also try to accomplish a real task (not just proofreading the docs, but actually using the docs for their intended purpose).

I've used docs from Salesforce and Google and can tell they haven't been tested in this way since they contain (glaring) problems that would easily be found and addressed in a few minutes.

I use this approach often, and gain enormously from it.