How do I tell if a contractor built my driveway to specification?
There is really only one way to check the construction of your driveway.  You will need to select a couple of locations and using a shovel, remove the top layer to expose the base layer, then remove the base layer to the grade fill and measure the layers.

If you are not comfortable that the driveway was built to spec, notify the general contractor or party you are purchasing the house from, as well as your lender and refuse to close until the driveway is properly built and proven to your satisfaction.  Stand tough, once you close it will be much more difficult to get this fixed. Insist they prove to your satisfaction the spec was followed.  Good Luck