The scariest part of this recent news is that TM Signal seem(ed) to be interoperable. People using TM Signal could interact with actual Signal users. How are you to know whether or not your groups have people using bastardized versions of Signal? Are things like Session interoperable with Signal?
The direct answer to your question is: verification of the security of the platform that the other party is using is outside of the scope of the Signal protocol. Anything you send to the other party can be taken off of their device. Signal only concerns itself with securing the message over the network and making it hard for an adversary with network dominance to build a social graph. It doesn’t protect from all SIGINT.
Additionally, since the server is open source and the protocol is open an publicly documented, it is completely possible to build your own Signal client and give it whatever capabilities that you’d like.
There are several open source packages available that allow you to interface with Signal without using the official Signal client:
https://github.com/AsamK/signal-cli
https://gitlab.com/signald/signald (also, https://signald.org/articles/clients/ )
Those third-party clients have some essential, basic functionality that the official ones for some reason lack. Signal-cli allows registering from desktop without any smartphone, Molly allows an arbitrary Socks proxy instead of being limited to just Signal’s own proxy solution, tying a desktop client with a link instead of scanning a QR code (thus allowing easy registration from an Android VM), and maybe most importantly for some - Notifications not relying on Google (Molly-Socket allows it to use UnifiedPush).