Here is a rundown example of how validation occurs in the Ethereum 2.0 (Serenity) Consensus Protocol.
There are several specifications that are required that define that elements of Serenity. I will be explaining only a few but important key elements that are used in PoS.
The Beacon-Chain, sometimes known as the 'beacon-node' or 'master-node' is responsible for lighting up the nodes in the network.
Nodes are classified as either validators or attestors. A random choosing mechanism is used to determine which node becomes a validator and the rest becomes attestors. This is calculated based on a form of weighting, in this case, the more amount of ether you put in the more likely you'll be chosen to propose a block. Blocks are not 'forged' like in PoW but rather 'proposed' by a validator.
Once a block has been proposed by the validator, it gets sent to the beacon-chain and is then broadcasted to the attestors who must vote in their time slot (I will explain this in sharding).
The validation comes from the attestors and the block is accepted, the block also needs to pass many pre-conditions, similarly to PoW with current hash meeting parent hash, time is greater than... you get the idea.
With sharding, there are different groups of validators that propose different blocks. In Prysmatic Labs Eth 2.0 Implementation they use 1024 shard chains in which validators are continuously shuffled.
For example, each shard has 4 validators, 1 of them gets chosen to be the validator and the last 3 becomes attestors. The validator proposes a block and that block gets attested by the 3. Repeat this for 1024 shard chains and you'll end up with 1024 proposed blocks in which attestors can vote on to see which block gets pushed to the blockchain.
Attestors vote either whether the proposed block should be pushed onto the blockchain, There are a couple of rules that are implemented to counteract malicious actors that may try to vote for blocks that could mess up the network.
Attestors are pushed to vote on the right block as if they vote for the wrong block, vote on multiple blocks or vote out of their time slot, their eth staked will then be slashed from the network.
A time slot is given to each attestor to vote on the proposed block, this is done to reduce malicious actors from trying to compromise the network.
There are many more layers, protocols, rules and procedures that will be used in Serenity. You can find out more here at:
Github Ethereum 2.0 Spec - https://github.com/ethereum/eth2.0-specs
Eth Research - https://ethresear.ch/
See ProtoLambda's tweet here, which shows an Eth 2.0 schematic that is out of this world. https://twitter.com/protolambda/status/1090272649421508609