To detect when a weapon is fired you can use an event handler like so
player addEventHandler ["Fired", {
params ["_unit", "_weapon", "_muzzle", "_mode", "_ammo", "_magazine", "_projectile", "_gunner"];
if (_weapon == secondaryWeapon _unit) then {
shotsFired = true;
};
}];
This will set the shotsFired variable to true when a player fires any launcher. You can limit this to a specific weapon by replacing secondaryWeapon player with the classname of the weapon.
If you want this to activate your trigger, put:
!isNil "shotsFired"
into the condition field.