How do I check the build or version number of VMware ESX?

The ESX build (version number) information is available in the Summary tab of the vSphere Client, but in larger environments it is worth to use some kind of automation. PowerShell/PowerCLI is well know scripting tool for VMware vSphere.

Below is PowerCLI one-liner to easily query all vCenters where you are connected ...

Get-VMhost | Select-Object Name,Version,Build 

If you want connect to vCenter(s) interactively, you can use following script ...

# Connect to vCenter
Write-Host "Connecting to vCenter ..."
$VC = Read-Host "Enter one vCentre Server or multiple vCenter servers delimted by comma."
Write-Host "Enter vCenter credentials ..."
$CRED = Get-Credential
Connect-VIServer -Server $VC -Credential $CRED -ErrorAction Stop | Out-Null