If you switch to the advanced thermal analysis type you get a lot more result control options. This gives you the ability to measure how much heat is lost from of each surface.
I’ve made a copy of your project and set up a example simulation for you. I’ve not checked if the results make sense but it should at least demonstrate the idea.
Is this what you are looking for?
Here is a plot of temperature and heat flux. It looks like there is a problem with the mesh under the heat sink. This can probably be fixed by applying a mesh refinement to the PCB under the chip.
I hope this helps.