When buying online, how do I know the exact CPU type?
Though Apple doesn't provide the info on their website, there are a number of tools that allow you to easily find what CPU is being used.

Let's take a look at the highest configuration of the iMac 27" 5K for our example.



Apple's website lists a 3.3GHz quad-core i5 and a 4.0 GHz i7. Viewing ark.intel.com, we can see the current Skylake generation (6th gen) of processors has two processors that align with what's in the iMac: the i5 6600 and the i7 6700K. 

However, this method requires some guesswork and a little bit of searching. There are easier methods.

The app MacTracker provides a slick interface for viewing the specs of almost every Apple product to date. Looking at the entry for the 5K 27" Late 2015 iMac, we can see the available CPU options.



If an app isn't quite your style, everymac.com has a list of almost every Mac with specifications as well.