When I ride with my heart rate monitor, my Garmin will tell me that I'm burning (typically) 600 calories an hour. When I ride without my heart rate monitor, it tells me that I'm burning (typically) 900 calories an hour. I'm fortunate enough to have a medical where my V02 max (one measure of endurance fitness) is measured, and that factor of 1.5 is quite close to the ratio between my VO2 Max and the average general public.
That might be one way of measuring fitness as a benchmark, without the need for labtests. I'm also aware that there are other standard tests such as the ramp test for measuring fitness, but I've not had to suffer any of those.