Jim,
If you are in the LDE, hit the \"M\" button on your keyboard. This will show you all of your snap-to points. But, like you said, on the doors you can only get the over all dims.
I case you are unaware of this option: If you set a door up as a \"build in house\", you will get sizes of the individual parts in the cut list. Make sure you set your \"default 5 piece door details for cut list\" settings to match the way you make your door. This is found under settings/preferences for a global setting or can be set specific per door. Hope this helps.