I saw a post somewhere where a person posted a text in Folksprak to see if people could make sense of it. This got me thinking; how recognizable is Esperanto? Yeah, its a posteriori, but so is Folksprak, and no one speaker of any Germanic language could make much sense of it. In fact, Esperanto's recognizability may be even worse since its derivation infamously renders some rather alien words. A common example I've seen is how the word for 'school' is 'lernejo', even though all the words for 'school' throughout Europe bear some resemblence to each other. If you weren't told before, why would you assume that 'lernejo' meant 'school'? Maybe someone would realize it had some relation to the word 'learn', but that doesn't help too much. Also, who on earth would assume 'malsunulejo' means 'hospital'? You get the idea. Even then, there are some seemingly a priori roots in it, such as famously 'edz-'. Those are rare granted, but they certainly wouldn't help.
Even the fact that its based on European languages may not be all that apparent. Given how so many non-IE languages have borrowed huge swaths of vocab from English, all the vaguely English-like roots may not be enough to give away that its even based on European languages. It may simply mean nothing to a casual observer.
Is there even anyway to test for this given Esperanto's fame? You find any material in it, its learning material. You're not going to run into a paragraph of Esperanto that doesn't explain to you what it means. Has anyone ever stumbled upon Esperanto text and not realized what it was? Maybe they didn't know Esperanto existed, but what might they make of the text?