Determine your VO2 levels. This will usually require monitoring in a facility, since you would need access to expensive equipment that measures your oxygen intake during exercise. You can also measure your VO2 Max to find out your maximum aerobic fitness level.
Divide the measured VO2 level by 3.5 to calculate the number of METs that figure represents. As an example, if you measured 28 mL/kg/min, then you divide 28 by 3.5, which gives you the measurement as 8 METs.
Compare your number of METs to required levels. Some professions, such as firefighting, require a level be achieved. As a firefighter you may be required to have a MET level of 12, for instance.