We can check the ESXI version using the following three ways.
1st ) From Vmware Vsphere client
Login into the Vspehre client using the ESXI node ip and root login details, once you logged in you will receive the ESXI version in the top bar
2nd) From Vcenter also you can check the ESXI version, login into the Vcenter using the Vcenter Server ip and admin login details and click on respective ESXI node IP from the node list present in vcenter , for example click on 192.168.209.13 node and check the ESXI version details in the following snap
3) If you are technically sound than you can also check the ESXI version from the shell , login into the shell and run the command
It will show you the ESXI version, for example check the following snap
For more information Vmware keep watching Vmware section