proof of end to end encryption

For end to end encryption is it possible to definitively verify that it is private? I think that because it states it is end to end and even gives private/public keys that does not mean that something such as whatsapp cannot still have unapproved access. I am not educated in computer science or software engineering but am wondering if end to end encryption through sources like Whatsapp and Telegram are truly trusted even as closed source software. Can the answer …

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