For starters there is
player_menu_info() which allows you to check if a menu is being displayed to the player, and which one it is.
But I don't think that is your problem, your problem is probably that you don't correctly handle your menu being closed. When another plugin overwrites your menu, your menu callback is fired with MENU_EXIT (as you can see
here and
here). Your plugin handles that event and probably destroys the menu. It makes no sense to call
menu_destroy() again on the menu, so this behavior is expected.
Unless you can show me a test case where the menu really is destroyed without your plugin noticing it, I don't believe this is an issue with AMXX.
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