How to predict new node allocation in a Cassandra cluster

1. You can use check Java Heap memory for each node. Like Total Java Heap Memory and Using Java Heap Memory.
2. One most important is CPU Utilization for each node.
3. Set alerts for errors. system.log have lots of information about.
4. You may set alerts for data disk and log disk.
5. Heartbeat check of servers like if you dont receive in few min it will get alerts to you.
6. Also dropped mutations and hinted hindoff clear alerts.

Basically you need to start observe system.log and will get more and more error for monitoring.

I initially just wanted to point at this as a comment, but my rep is too low. So excuse me if this is not a fully fledged answer. (Anyone feel free to edit or hint in comments)

Looking at the documentation at datastax it appears that data will be evently divided by nodes, depending on the "automagical" tokens. From my understanding it's simple as that a new vnode with new tokens just takes an even portion of each node, like all the other nodes already do. Hell, this is even what the doc says basically word by word:

Rebalancing a cluster is no longer necessary when adding or removing nodes. When a node joins the cluster, it assumes responsibility for an even portion of data from the other nodes in the cluster. If a node fails, the load is spread evenly across other nodes in the cluster.

And to address your question:

To further specify my question for virtual nodes does the cluster re-evaluate tokens for all nodes just because a new host has been added?

Sort of. The new node itself takes some partitions (evenly) from all other nodes. If you remove a node every other node will rebalance its partitions from the other nodes, making up for the lost node.