http://whirtorrgetftvz4g466sjqkegtyi35bjl4bvotfkfossunf5my4x6ad.onion/bitcoin-white-paper
He can't check the transaction for himself, but by linking it to a place in the chain, he can see that a network node has accepted it, and blocks added after it further confirm the network has accepted it. As such, the verification is reliable as long as honest nodes control the network, but is more vulnerable if the network is overpowered by an attacker.