I agree that the open-source ability to audit is one of the strong points of foss and that's an allure to keep everyone honest. Look at Signal, ProtonMail, Telegram, and to a lesser extent WhatsApp, AWS and Zoom. Once things get increasingly proprietary, they tend to pull the curtain wider across the stage.

Network traffic - or rather lack thereof - could conceivably be a good place to start. Wireshark not showing you any tangible information when you're sniffing the data? You stand a good change of being positively encrypted, but that's only one end of the end-to-end.

There's an interesting research paper from the 1984 IEEE Symposium on Security and Privacy written by Dianne E. Britton entitled, "Formal Verification of a Secure Network with End-to-End Encryption" - the abstract being:

A formal specification and verification of a simple secure communications network using end-to-end encryption is presented. It is shown that all data sent over the network is encrypted and all heats on the network exchange messages only if they are authorized to do so. The network and its hosts are modelled by a set of concurrent processes that communicate via unidirectional buffers. Each process is viewed as a state machine. The specification has been formally verified using the commercially-available VERUS verification system.

I feel like this would at least illustrate the groundwork current systems have expanded on over the last twenty-odd years, but the paper is behind a research wall. If I can get a copy, I'll link to it here.

The other aspect worth mentioning is Asynchronous messaging with as strong as security as possible - the paper that details this (https://people.cispa.io/cas.cremers/downloads/papers/CCGMM20...) uses the word "Guarantees", but is it ever? - the abstract for that being:

In the past few years secure messaging has become mainstream, with over a billion active users of end-to-end encryption protocols such as Signal. The Signal Protocol provides a strong property called post-compromise security to its users. However, it turns out that many of its implementations provide, without notification, a weaker property for group messaging: an adversary who compromises a single group member can read and inject messages indefinitely. We show for the first time that post-compromise security can be achieved in realistic, asynchronous group messaging systems. We present a design called Asynchronous Ratcheting Trees (ART), which uses tree-based Diffie-Hellman key exchange to allow a group of users to derive a shared symmetric key even if no two are ever online at the same time. ART scales to groups containing thousands of members, while still providing provable security guarantees. It has seen significant interest from industry, and forms the basis for two draft IETF RFCs and a chartered working group. Our results show that strong security guarantees for group messaging are practically achievable in a modern setting.

KeyBase and Wire (https://medium.com/@wireapp/making-your-conversations-secure...) are both doing interesting things with end-to-end encryption, but verification from a user perspective - especially in the case of proprietary software - leaves the onus on the user to simply stay as paranoid as (reasonably) possible for the time being.