Assumption is we know who's mining for which pool. Whenever a known pool / solo miner finds a block, he also builds this altcoin block. He uses same coinbase as in Bitcoin main chain, and rewards himself with 12.5 altcoin. We can check last 24h blocks to verify this.
How exactly would you verify this? And how would you prevent the largest pools from bypassing this scheme by splitting up, or by pretending to split up?