Calculate how much heat an IC produces
Here is rather simplified answer: with few exceptions (like light emitting or RF radiating) the electronic circuits convert all incoming power into heat. So, if you measure the power consumed by your assembled device you can get pretty good approximation of the heat to be dissipated.

Of course, if you want to calculate it in advance or predict temperature in various conditions you need all those things described in @SpehroPefhany's answer.