To find how much exogenous carbs you are absorbing and therefore able to use you need to put a tracer (in this case a glucose and fructose carbon isotope) and monitor how much of those isotopes are you exhaling in the co2 you breath out. I would imagine there is also ways to do this with imaging of the traced carbs in the stomach vs intenstine like Maurten has done in some of their research. This is extremely expensive and complex so good luck haha.