For someone who knows that these tools exists, yes this is a way.

For an ordinary user, the only 'easy' way a user can verify the claimed behaviour is to literally go offline.

Browsers do not currently have badge to verify that the app is not sending any data. I'm thinking how we were brought to trust the padlock icon browsers display for TLS supporting sites.

Something like that would be great, seeing that you can do more and more with wasm locally it would be really useful