Determining whether :update actually saved file?

Is there a way to call :update <some other file name> such that you can tell whether there actually were changes and that the file was written?

I'd like to perform an action each time the current buffer changes (without explicitly saving to the current file), but presently calling :update <other file name> seems like a black hole as there's no way to know whether anything was written or not.

I've tried treating update as a function by doing things like `echo update`, but it seems that it behaves differently from a function and it errors...

Thanks,

Suan

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