To detect if the player has a weapon in his hand, you just need to check if his behaviour is COMBAT, AWARE, or STEALTH. Create a trigger, any size, activated repeatedly, and activated by anyone. The put this line in the "CONDITION" field:
(behaviour player == "COMBAT") || (behaviour player == "AWARE") || (behaviour player == "STEALTH")
And put what ever you want in the "ON ACTIVATION" field
This method should work, but i'm not 100% sure