SPICE IC model in-circuit power dissipation
The power can be estimated by summing all of the power flowing in and out of a part (because energy is conserved). This doesn't work well for some models that have voltage or current sources built into the model (that can magically dissipate and create energy).  

Typically power can be calculated for an IC by looking at the current and voltage going in and out of all the pins (this is what LT spice does if you alt-click on a part, but it will add all the signals from an IC to calculate power dissipation automatically). 

Keep in mind that even after the power has been calculated, there are further steps to finding the actual temperature of the part, like the thermal resistances and average dissipated power