A Monero block does not say who mined it: the coinbase output goes to a one-time stealth address, unreadable without the wallet's private view key. Attribution has to come from somewhere else.
Proven by view key
Some pools publish the view key of their payout wallet. With it, anyone can check cryptographically that a block's coinbase output belongs to that pool. This is the strongest form of attribution.
Listed by the pool
Most pools publish the list of blocks they found. We match each block hash against those lists. This depends on the pool being honest and on its API being up; a pool's list can also arrive late, so the most recent blocks may be attributed with a delay.
Unattributed stays unattributed
Blocks that match no known pool are not guessed. They are counted as unattributed, and they stay in the denominator of every share, so no pool looks larger than it is.
See the result on the pool distribution page, or check a single block, for example the latest block.