I use an addon - Shadowed Unit Frames. It has a resurrection icon that is displayed while res is being cast. It accomplishes this by using the api function UnitHasIncomingResurrection. Unfortunately, the icon vanishes when the res cast ends.
It is unsuitable for determining if the defibrillator worked.
I have reviewed the api, and the simplest way an addon could accomplish what you're asking for is
There might be an addon that does something similiar to this, called AutoRez.
The only way to tell if any resurrection attempt has been successful in World of Warcraft is to ask the person you were trying to resurrect. This is the same for all forms of resurrection, be that a casted resurrection, combat resurrection, or one of the other forms of resurrection as an effect of on use on items (as per your question).
In all instances, the target for the resurrection will simply receive the "You have been resurrected [Accept/Cancel]" box but there are no further indications than the resurrection attempt was successful.
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