If it has stock cooling head, you can tell by the profile of the head...
The pro.15 has 4 bolts holding it on, and has a 'normal' profile...It is straight up and down, and the fins are evenly spaced.
The 2.5 has 5 bolts in the cooling head, and its profile is bulged in the middle, and the fins aren't all evenly spaced. (also has little plastic do-hickey on the top of cooling head, so you don't scratch it up when you flip it over...or at least it did until somebody flipped it over...)