Proof of end-to-end encryption

Is there any way for a service, let's say a messaging app, to prove that the data going through it is end-to-end encrypted?

Forget about man-in-the-middle for now. And forget about open source and self hosting. And we don't trust independent security audits either. Also, let's say I don't want to add extra steps to the usability of that service, like additional encryption from my side. It should be an out-of-the-box feature.

Is there a way to truly verify if …

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