Turn the steering from lock to lock withstand staitionary, or better still, going slowly in reverse. And clinker, probably bushes. 159 2.4 is a heavy hector, and will strain bushes.
Check for shot blasts of stone chips on the lower rear arches and side skirts. Those lovely wheels pepper the bodywork.
The brakes should be firm, without a great need for much pressure. Again a heavy car, needs the brembos in mint condition.
Check colour of the oil. Thick black oil is not good. And ignore the service interval of 20k would you really want to run a high performance turbo engine on oil for 20k?
Put the car in 4rh at 30mph and then floor it, and watch for clutch slip or judder. This would he expensive. 2.4 Is very torquey, so clutch will take a lot of beating in tall gears.
Walk away from a car that has been re mapped. As would be better to re map yourself, and then drive it hard. You know then how it has been treated after having its performance improved. And you will drive away the hard, as re mapping the 2.4 is the way forward!
All alfas I have owned, with big wheels, have suffered from wheel wobble at 70+ mph. Alfas are very sensitive to wheel weight losses and tracking being knocked. Look out for it, and budget for a quality 4 wheel laser alignment. Changes the car dramatically.
The 159 is a glorious machine. Good luck with your search, and enjoy it.
'07 GT Blackline JTD, but a 159TI 2.4 on its way very shortly!