Cassandra cluster monitoring
You can use check Java Heap memory for each node. Like Total Java Heap Memory and Using Java Heap Memory. 
One most important is CPU Utilization for each node.
Set alerts for errors. system.log have lots of information about.
You may set alerts for data disk and log disk.
Heartbeat check of servers like if you dont receive in few min it will get alerts to you.
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.