During VMworld this year, I attended at nice session on vSAN troubleshooting tips. There are some tools that help us to find a root of problem:
- esxcli
- RVC
- Health Check
- vSAN Observer
- vRealize Operations Manager
- vRealize Log Insight
esxcli
There are some commands as below examples:
- esxcli vsan network list - Verify which VMkernel adapters are used for vSAN communication
- esxcli vsan storage list - List storage disks claimed by vSAN.
but quickly we can check health of vSAN cluster by running the following command:
- esxcli vsan health
Ruby vSphere Console (RVC)
RVC is a useful tool (an interactive command line) - as part of vCenter Server (Windows and the vCenter Server Appliance) to manage or perform troubleshooting on vSAN cluster.
Health Check
This is a plugin available in vSphere Web Client.
vSAN Observer
- Monitor performance statistics for a vSAN Cluster.
- Provide access through a web browser.
- Capture the statistics for customer use or for VMware.
vRealize Operations Manager
There is vSAN Management Pack for vRealize Operations Manager (embedded since version of vROps 6.6).
vRealize Log Insight
vRealize Log Insight is a powerful syslog that can show (vSAN Content Pack) anomalies in logs taken from vCenter and ESXi hosts. 
Please remember that limited edition of vRealize Log Insight is included in vCenter license.
Useful resources:
VMware ® Virtual SAN Diagnostics and Troubleshooting Reference Manual
Using Esxcli Commands with vSAN
Using vRealize Log Insight Content Pack for vSAN for better visibility
Managing Virtual SAN with RVC: Part 1 – Introduction to the Ruby vSphere Console