For open source messengers, you can check whether they actually encrypt your messages and whether the server has access to your encryption keys but what about WhatsApp? Since it's not open source, you can't be sure that the encryption keys aren't sent to the server, right? Has there been a case where a government was able to access WhatsApp chats without reading them from the phone itself?
No. They cant decrypt your chats. They can however backdoor your device and see the pre or post delivery message. Its not hard for them to do. Technically or legally.
If they arent currently logging activity on your device then turning on self destruct messaging could mitigate their ability to spy on you. Unfortunately all your chat partners have to do it too.