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Many different PoS approaches have been proposed but I am having a hard time finding documents that explain exactly how their protocols work. So, what are some current implementations of PoS?
Where can I find documentation about how PoS validates blocks?
For Ethereum there are really two different implementations of PoS.
FFG (Friendly Finality Gadget): This is what people will refer to as hybrid PoW/PoS. Everything works the same way it does now, with miners mining blocks, but there will also be validators that vote on what block to finalize. Finalizing is an addition to the longest chain fork-choice rule where nodes refuse to revert a finalized block, meaning it's impossible for a 51% attack to undo more than x blocks (an epoch). More reading can be found here for basics and here for economics
TFG (The friendly GHOST): This is what people will talk about when they talk about full PoS, so it isn't overlaid on top of mining like FFG is. More reading can be found here and here
The test net is up - here is the link to the setup, which includes references to white papers published: https://hackmd.io/s/Hk6UiFU7z#.
Here's also a post about the latest issues with the test net: https://www.reddit.com/r/ethereum/comments/7q2ysr/casper_testnet/dsmkb3f/?context=3&st=jcrw681z&sh=5b59f012.
There's also a youtube channel of the ongoing research: https://www.youtube.com/channel/UCi8byRkpJBbGgDot2pWXLHA/videos.
You can join the developer chats: https://gitter.im/ethereum/home
Ask the developers directly or those working with them. That is where I get the best source of information.
Follow the ethereum team on social media, follow the ethereum blog and take a look at the code commits on ethereum github projects.
