Watch the traffic go by with a packet sniffer? Verifying that it is done securely is a lot harder, but you can at least verify that things are not being sent as plain text/data.
Watching traffic is irrelevant, the encryption of the transport (WhatsUp always used TLS for that) is not in question here, this is about end to end encryption and if WhatsUp can either decrypt the data for users or switch them off E2EE at any point without it being noticeable to either end.
Yeah but that just shows its https.
Can't you MITM yourself fairly easily to decrypt the https and see what's being passed? Assuming you have full control of your device.