Merkle Proof Validation
Specter-Desktop validates BIP 37 Merkle Proofs from your full node guaranteeing that transactions displayed as confirmed have been included in the specified block hash.
However, a compromised full node could return a fake block that was never mined into Bitcoin's blockchain!
If you find the displayed block hash in other locations (other nodes, block explorers, etc) and you trust specter-desktop isn't lying to you, then you can be sure this transaction exists in their blockchain as well.