Cap Rate

[Simon Cox]

Hey everyone,
I'm having a bit of trouble determining the proper use of a commercial property's cap rate.

I know the basic calculations that describe where it comes from but my question is with how do you decide what cap rate to use in calculating fair market value? Answer(?): from comparable properties, but what if there aren't any? 

When finding the average cap rate for an area, do you use the cap rates that go along with the asking price?

Any other insight on the topic would be greatly appreciated. 
Like I said, I get the main idea but am having trouble putting the rubber to the road on how it is actually used.

Thanks everyone!