While being accurate about it is hard outside the lab it is very easy to tell where you are on the balance and how much out you are. Just count the calories you consume and weight yourself regularly. If you are gaining weight then you are eating too much, so lower the number of calories you are consuming, if you are losing weight then you are eating less than you are burning. If you weight remains stable then you are in balance. And the amount you are gaining/losing tells you how much of a surplus or deficit you are in.
Over time you can then change the amount you eat by I few hundred calories at a time and you will see yourself move on that balance point. If anything else changes but your intake remains the same then it is likely your calories out that has changed. But even if technically you are digesting less for some reason it does not really matter - the bigger/easier leaver you have to pull is the number you are eating.
Because you are measuring the final output - your weight - it is fairly accurate over time and helps you track actual progress. There is no need to get super accurate about how much your body adobes, shits out or you burn off at rest or through exercise - those might be important in the lab but in real life the far easier to measure weight and how much you are eating is more important.