A rough estimate of your VO2 max can be calculated without undergoing physical testing if you have an idea of your resting heart rate and maximal heart rate.
Maximum heart rate is strongly related to age, you can calculate your theoretical maximum using the following equation:
max heart rate = 208 - (0.7 x age)
This equation was found to be more accurate than the more common 220 - age approach in this study by Tanaka et al.
Next, you'll need to measure your resting heart rate - Put your middle and index finger to either your radial artery on your wrist or your carotid artery in your neck. Once you find your pulse, count how many beats occur in 20 seconds, and multiply this number by 3.
VO2max = 15.3 x HRmax/HRrest