Paste the runtime code and creation code a counterparty gave you, plus the deployer and either a nonce (CREATE) or a salt (CREATE2). The node hashes both blobs, checks the two size limits, reads the fields the compiler appended, recomputes the address the deployment must have produced and compares it with the address and verified-source hash they declared. Nothing is read from a chain, so it tells you whether the bytes agree with the claims, not whether those bytes are what is deployed.