Using Value Preview, you can see the value of each part of a formula in Excel.
Just highlight the reference you are interested in, and a little box pops up that show you the value(s) in that reference. Or you can click on the parameter in the formula structure.
More information and some videos showing it in action here.
The other way you can do a similar thing is to use Evaluate formula on the Formulas menu (Shortcut Keys Alt, M, V).
This allows you to step by step work through the formula and see how it is being worked out. Press the Evaluate button to work out each step one by one. This can be very handy if you have a formula that isn’t working, and you are trying to see why.
So, use Value Preview next time you are working with a formula.