I am not sure what you mean by UAe and UAn. But I am going to assume you are referring to the aerobic threshold and anaerobic threshold.
For Aerobic threshold, a gas exchange test will give more precise data as to the fuel source but a lactate test is also a good way to find Aerobic Threshold.
For Anaerobic Threshold I recommend a field test like described here: https://uphillathlete.com/diy-anaerobic-test/
Scott