When and how can understand my esxi host is in isolated mode ?
Host is isolated if it cannot ping the isolation address (usually the default gateway) and the vSphere HA agent cannot communicate with any other host in the cluster. Read more about that in the following documentation link: vSphere HA Agent is in the Network Isolated State
As Rich said, Isolation address your default gateway address and if management network failed to communicate your hosts will be in isolated state but your VM's will be up and running till the time you have the storage heartbeat.
You can change the default isolation address. Please refer below articles,
How do I use das.isolationaddress[x]? - Yellow Bricks
