Thank you guys! I finally solved it this way:
Iterator it2 = (Jenkins.getInstance().getItems()).iterator();
while (it2.hasNext()) {
AbstractProject job = (AbstractProject) it2.next();
Object instance = (my-super-duper-plugin)
job.getTriggers().get(this);
if (instance != null) {
// This job uses my-super-duper-plugin
}
}
Being this a Descriptor.
