Well yeah, if you have a dead person that's staying dead (i.e. no ROSC) you can't stay on scene for the rest of time beating on their chest, eventually you have to pack up and clear and go back in service, so yeah you're gonna have to tell the family that they (the patient) is dead. Why wouldn't you be able to state a fact like that? However, just remember to use Tact when in that situation..
The vast majority of cardiac arrests on the field simply don't survive. Most services are going away from transporting every cardiac arrest, instead only transporting once pulses have been regained. So if you find yourself working in a 911 environment you're gonna have this scenario happen more often than getting a "save".
As far as knowing when to call it vs when to stay and play, simply follow the protocols your medical direction wants you to follow...when in doubt call your Online Medical Direction..