Waste power is heat to first order.

If there's a lot of it and you're not making it go elsewhere, assume that the system and it its environs are going to get a bit toasty.

Sometimes you also get things like radiation and vibration. These mostly show up in meaningful quantities in predictable places - nuclear systems, mechanical systems, high-power A/C or RF systems, high-power transducers.

You'll usually do okay if you guess that the system uses energy at around ~60% efficiency, which is enough to give you some idea of the heat to expect if you know the input power.

Or more generally, assume all input power goes to heat and plan to handle operation at that limit - while being aware that your expectations of power consumed by the system may well be low.