how to know sender of triggerenter event?

I would need to know the sender of a triggerenter event. I mean the collider which fired it. I know it is quite easy to script this, but now I need it for debugging. I have two monsters with the same setup and one of them does fire these triggerenter events but the other does not. I would like to know why the first one fires the event, because it should not be fired as far as I know. Could anyone help me please?