To find out if a plugin or command is active, there seems to be no real API functionality. What can you do to see if the plugin is loaded, use
var plugin_is_usable = tinymce.get(editor_id).plugins.pluginname;
To check if this command is available (in this case mceAutoResize), you can use
var mceAutoResize_is_usable = tinymce.get(editor_id).execCommands.mceAutoResize;
It doesn't look like this will ever change in tinymce.
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