This question has been answered by markh. See the first response.

http://domain.com/manager/index.php?a=13&id=4
I'm pretty sure there's a way to get the name of a manager action based on its ID (and vice versa). Mark Hamstra told me how to do it, but I can't remember what it is. I sent him a tweet, so hopefully he'll chime in and we'll all know.I think I understood that I can check $controller->config['controller'] or get it by $modx->getObject('modAction', $actionId) and if it is needed to me then do with this page anything. But interesting what Mark will answer then.
10 =>
array (
'id' => '10',
'namespace' => 'core',
'controller' => 'element/chunk/update',
'haslayout' => '1',
'lang_topics' => 'chunk,category,propertyset,element',
'assets' => '',
'help_url' => 'Chunks',
'namespace_name' => 'core',
'namespace_path' => '/Users/sottwell/Desktop/Working/htdocs/revo226/manager/',
'namespace_assets_path' => '{assets_path}',
),
if (array_key_exists($_GET['a'], $modx->actions)) {
// Only do something on resource/update
if ($modx->actions[$_GET['a']] == 'resource/update') {
$modx->regClientStartupScript('foo/bar.js');
}
}
if ($_GET['a'] == 'resource/update') {
//foo bar!
}