That used to be the case on Reddit, not any more as about half of all accounts start off shadowbanned and mods approve their posts.

It would be one heck of a challenge, but, almost by definition "impossible" - as a naive guess, perhaps (for those platforms that have opened code) characterize some statistical signatures that "arbitrary data" ran through it (simulating user activity) would show ...

... and then check for those (or, counterexamples) in the wild.-