ok...
now as there is some talk about serious methods, you can:
1) destroy the cached file
2) look for it's creation
3) once created, try to delete it
4) when delete is possible, the page has loaded

anyways, you can create whatever methods to make sure the page is opened but I can't understand why you would want to make it 100% sure.
then, how would you control user scripting it to open in hidden window?
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