Public services and E2E Encryption. Is there any truth to it?

Lately I was thinking about companies who claim to use End-to-End Encryption (WhatsApp. Messenger, Telegram if you enable it, Signal, etc.).

We can't really prove our messages are E2E encrypted, can we?

Have there ever been any incidents with these apps where this kind of encryption failed?

apps claim companies e2e enable encrypted encryption end end-to-end etc incidents messages messenger privacy prove public public services services signal telegram thinking truth whatsapp