Since fusion-Rendering is meant to be side effect free there is no good way to decide such stuff from the code that is rendered in the main collection.
The Neos way to do this is to add the script to the head section but make sure that it is included only if the content that needs it is present.
javascripts {
contentScript = ...
contentScript.@if.contentIsPresent = ${q(documentNode).children('main').find('[instanceof Vendor.Site:Content]').is()}
}