How can I check in the map editor who has captured a building and perform a specific task if they are captured by different factions?

[SpartakZ]

How can I check in the map editor who has captured a building and perform a specific task if they are captured by different factions? I want to do something so that if I captured this building, the mission would be completed, but if it was destroyed, then I would lose. I want to do this for an online game.