Get-OSTypeFromTemplate returns the VSphere API 'ostype' from a defined template. This can be used to build out an OS Customisation Specification when deploying a new VM from template. Easily extensible for other OSes or more specific flavors. If the template does not give up its secrets, the script stops and waits for you to type it in.

Get-OSTypeFromTemplate.ps1

Function Get-OSTypeFromTemplate {
    param($templatename)  
     
    $templateview=get-view -id (get-template -Name $templatename).id
    switch -wildcard ($templateview.Guest.GuestFullName) 
    { 
        "*Windows*" { $ostype = "Windows" } 
        "*Linux*"   { $ostype = "Linux" } 
        default     { $ostype = Read-Host "Unable to determine OS from template $templatename. Enter the word Linux or Windows here" }
    }
    
    return $ostype
}

#Add-PSsnapin VMware.VimAutomation.Core
#Connect-VIServer $vcserver

$templatename = "RHEL 6.2 x64"
$ostype       = Get-OSTypeFromTemplate -templatename $templatename
$ostype       # "Linux"