Most Colnagos have Columbus decals on them which provide a big clue. I think the newer ones are made from Columbus Foco, Ultrafoco, Genius, or one of the other newer steels. The weight of the frame will also give you some idea; the new ones are no where near as heavy as the older ones.
I was in a similar situation a few months ago, and I simply took everything off the two second-hand Colnagos I was looking at, and weighed the frames (totally bare, except for the head-set cups which weigh about 30 grams, if they're alu), and they were absolute ANVILS! They both weighed about 4.8lbs without forks, seat tube bolts, cage bolts.........no nothin'!!
I know weight isn't everything, but there's no need to hack around on a five pound frame in this day and age
