lunaticoin on Nostr: Theoretical exercise: Lets imagine that you are put in a bunker, with no TV access, ...

Theoretical exercise:
Lets imagine that you are put in a bunker, with no TV access, no social networks, no internet mempool explorers or anything.
But you have a Bitcoin node with internet capabilities only to receive blocks/payments and make queries.

How would you know that what you receive is real #bitcoin?