Determining wounds caused by specific weapons.

[TechnoVampire]

Hello

Several rules/ effects trigger when an enemy model is slain by a specific weapon such as Mannfreds sword of unholy power ability: 

“If any enemy models are slain by wounds inflicted by this model’s Gheistvor…”

Since wounds are calculated after all attacks have been made from multiple weapon profiles, and the models removed afterwards how do you determine if a model was killed by a specific weapon?

Thanks!