Should look something like:

$objDoc = $Word.Documents.Open($strFilePath + $strFileName)
$objTemplate = $objDoc.AttachedTemplate
$dlgTemplate = $Word.Dialogs($wdDialogToolsTemplates)
$strPath = $dlgTemplate.Template

and then, your going to have to find the enumeration value for this:

$wdDialogToolsTemplates = <somenumber>

find the excel/word object browser good for that.