Truly smart devices would choose not to connect to the internet.
It's the only way to be sure.
My Siemens Dishwasher kinda chose to not connect to the net.
I did connect it I have to admit, because I could DL and install some extra washing programs, but then Siemens did some update and since then it does not work anymore. I decided to treat that as a feature and did not try to fix it (although it is probably easy by creating a new account or such)
Or at least disconnect themselves