What sort of verification is done on a bitcoin transaction? This is because the transaction has already been hashed by the miner. So how can other nodes in the network say that the transaction is valid when they cannot see the transaction contents?
A Bitcoin transaction is validated against the consensus rules. This includes that any inputs in the transaction have not already been spent (they are valid utxo's used as the inputs) and several other things.
The contents of transactions can be seen but the transaction outputs cannot be unlocked to be spent without the correct key to sign for the receiving address.
You can have a look on blockchain.info if you are interested and look up transactions, addresses or, blocks.
You can find a very good Bitcoin primer in this video: