On the ESXi host:

esxcfg-info | grep -w "Serial Number"

Remotely via PowerCLI:

$myCredentialsObject = Get-Credential
Connect-VIServer -Server <;ServerName>; -Credential $myCredentialsObject
Get-VMHost |
Select @{N='VMHost';E={$_.Name}},
    @{N='Serial';E={($_.ExtensionData.Hardware.SystemInfo.OtherIdentifyingInfo |
    where {$_.IdentifierType.Key -eq "ServiceTag"}).IdentifierValue}}

If you have any questions/comments regarding this article, click here or scroll down below (login isn't required to post comments and there's no waiting period).