You should be able to:
While stationary with engine running, turn the steering with the use of your index finger pressing down on the spoke without bending your finger back.
When below 30 mph you can turn the steering easily with just both thumb and forefinger holding the wheel.
At speeds of 60 mph and above the steering gets progressively heavier for more precise steering corrections
Steering only takes 2.1 turns lock to lock unlike many other cars which are 3.0
Any heavy feel will be down to:
1. Power steering/ auxiliary belt removed from pump pulley i.e. used a shorter belt due to PAS pump failure... Walk away from the car, they have tried to hide the fact!
2. Lack of PAS fluid in rack and reservoir,top it up and turn lock to lock 10 times to bleed it. If it makes any squealing or weird noises after that, walk away from the car!
3. Spray plusgas onto the bottom of the rack steering U/J (universal joint) if the car has been standing on a forecourt for a while etc. It may have got stiff over time, if it doesn't improve, walk away from the car.
Last tip: get yourself along to the next local Alfa meet and I'm sure someone will take you out or even give you a quick spin in a well sorted and loved 156 that will confirm what you are looking for and exactly what to expect