ManagedArticle.unplacedComponents();

// Get the unplaced component GUIDs of the first managed article.
var articles = app.activeDocument.managedArticles;
if (articles.count() > 0) {
    var guids = articles[0].unplacedComponents();
    if (guids.length > 0) {
        alert("Unplaced component GUIDs: " + guids.join(", "));
    } else {
        alert("All components are placed.");
    }
}