There is some tools (or at least 1) but i think the best way is to use the built-in Function Wizard:
Select the cell with the complicated formula.
Press the fx button (left of the formula bar).
Now you should see the Function Wizard.
When you click in the Formula bar on (in) a nested function (or just move the cursor to it) (used as an argument) the Function wizard will show you the nested formula, it's arguments and there values.