Returns whether the specified character can cast the given spell right now. It checks whether he knows the spell and whether he has enough mana; it does not check anything else (whether he is silenced, whether he is dead, whether the room is a no-magic room, etc.)
The spell is given in the second argument exactly as if you were casting the spell in the game, i.e. 'tenebrous claw'; if the second argument is an integer, then it's the spell number.