I don't think they know what should be in a block. They only see what is in there after the block is forged.
But everyones sees what *could* be in a block, so it can be noticed if a node is shady
No. Not everybody sees what could be in a block, in fact nobody will see that in the not-so-distance future as nodes and clients will send transaction only to those nodes chosen by TF to reach the 1000 TPS.