When considering a slam, you first have to decide if your two hands have the power to take 12 or 13 tricks once you get the lead. You can often add your points to the number partner has shown to determine your chances.
Do we have enough points?
Do we have enough controls?
Your second concern is having enough "controls" -- aces, kings, singletons and voids -- to prevent the opponents from cashing two quick tricks. To bid and make a slam, your two hands must have: 
 
A first-round control (Ace or Void) in at least three of the four suits. 
At least second-round control (king or singleton) in every suit.