I usually use the WM_SETTINGCHANGE window message as a trap for background GP updates. One of the flags in this structure indicates whether it was caused by a policy or something else, so you definitely want to find it.
As to whether to use for each computer or for each user, it really ends with the type of application that you install with the policy. When Microsoft implements the same policy as for each computer and for each user, usually the settings for each computer benefit, but not universally.
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