Engineering is mostly about using assumptions and models to make predictions. For example, we predict the maximum demand on a circuit, then size the cable appropriately based on a predicted voltage drop, predicted temperature rise, etc.
The project often ends at this point. We’ve done the modelling and produced a result or a design. We get paid and move on to the next project.
Later on, the design (hopefully) gets built and used. The quantities that we predicted become real. We should go and measure them to find out whether our predictions were any good. The original project is unlikely to pay for this, so you should regard it as an investment, improving your prediction capability for next time.
Whenever you make a prediction, think about how you might validate it. If you are lucky, someone may already be logging the necessary data and you just need to persuade them to share it. In other cases, consider adding measurement equipment at the design stage.