Fat burning occurs when the supply of calories (especially from carbohydrates) that we consume in one day is burned out. So for example in the morning you have breakfast carbohydrates like fresh bread, then decide jogging half an hour with the best treadmill under 1000 in the day, then the body will take the calories from breakfast first before burning fat. If it is finished, then it will take from fat. This is where the crowds are not aware because modern people are very generous in consuming carbohydrates although the work does not involve a much physical activity or also called sedentary.
So it’s not when you’re sweating or just fainting that fat burning occurs. It will be difficult to detect when our practice has started burning fat. We must regularly monitor the comparison between total calories consumed with total calories used. The only two options are first, increase the time or intensity of the exercise so as to increase the total calories burned. Second, is to reduce (slowly) the number of calories consumed from food in a day so it is always lower than the total calories burned.