If you look at a car registered from 58 onwards it should be a facelift. Signs are,
rear badge is also a push button for the boot,
Heater controls that have blue and red bits above them.
Why is this important? From March in 2008 they lightened the front end. My understanding (I could be wrong) is that the lower subframe was switched to alloy and front roll bar became hollow rather than solid.
The alleged savings were c. 40kgs though there is suggestion Alfa used creative accounting to establish the weight saving.
N.B. They removed some of the electrical toys in the cabin on later models to reduce the weight also.
The wheels stick out on the 159 more than previous Alfas that I have owned, so expect kerbing on the wheels and lots of stone chips all along the flanks. The front end (though beautiful) is bluff and also suffers badly from stone chips. (not always indicative of life a in the fast lane)
They are heavy cars (Alfa lied about the weight) and tend to snap coil springs, listen for a graunching sound when mounting a driveway at an angle. A reliable replacement/slight upgrade (from Ti spec) is to change them all to Eibach pro sports, better quality, longer life and cheaper than the Alfa ones!
If you go for a saloon check that the boot lid does not hit the back window, the struts can break, but the main problem is the "over travel" tab which bends. This requires the boot lid to be removed and a welding gun to heat and reshape the tab.
Check the dash area around the key fob socket, because if people add their house keys etc. to the cars fob then they tend to dangle and scratch this area.
Ti is the best spec to go for and the most desirable, there should be a sticker on the drivers door pillar denoting the model, some owners have reported that their Ti's are Lusso on the V5
Edit: if someone is trying to pass off a Turismo or Lusso as a Ti (I have been to look at one) then be aware that you can buy these stickers on Fleabay.
Running report for: Type 937 Gt 3.2 V6
My Italian car history so far:
MK1 Fiat Uno 60 S, Alfa Romeo Alfetta GTV6, Alfa Romeo 156 2.0 ts, Fiat Multipla ELX 1.9jtd, Alfa Romeo Gt Lusso jtd, Alfa Romeo 166 3.0 V6 Super, Alfa Romeo 159 2.2 Ti, Alfa Romeo Gt 3.2 V6