If the car is equipped with the 3LT package, it'll now have radio controls on the steering wheel. This was not available in 2006. Also, if you see a two tone interior on a coupe/vert, that just became available in 07 too (you could get two tone interior on an 06 Z06 though). Another way to tell is if the car is painted in Atomic Orange, as that's new for 2007 as well.
You can also tell it's an 07 because there won't be a lock on the center console anymore, and the glovebox is bigger than the 06s too.
So basically most of the changes are things you'll need to look inside the car to see. So a car driving by you on the highway will be hard to figure out as an 06 or 07, unless it's Atomic Orange.