Check it yourself. And if it's been out of your sight or physical control, check it again. It's not considered rude if you don't take the other person's word for it. Don't let the last words heard be "It's not loaded."
I forgot where I read it, but the two loudest sounds you'll ever hear are a bang when you expect a click and a click when you expect a bang.
RULE II: NEVER LET THE MUZZLE COVER ANYTHING YOU ARE NOT WILLING TO DESTROY.
RULE III: KEEP YOUR FINGER OFF THE TRIGGER UNTIL YOUR SIGHTS ARE ON THE TARGET AND YOU ARE READY TO SHOOT.
RULE IV: BE SURE OF YOUR TARGET AND WHAT'S BEHIND IT.