mattwnz:
According to my Apple watch, my VO2max has improved a lot, but apparently it is not very accurate and is more just a calculation based on your activities. I am not sure how easy it is to get VO2max measured accurately.
A proper VO2max test involves running at increasingly high speeds on a treadmill with respiratory measurement equipment attached. It's very accurate but not a particularly pleasant experience.
Apple and Garmin watches generally approximate VO2max very accurately, as long as you are running on the flat at a reasonably fast pace. A running track is ideal, or an event like the Wellington Marathon. If you do a lot of trail running then it will have a detrimental affect on the accuracy. For reliable VO2max readings it's also important to ensure that your watch is getting accurate heart rate readings as optical wrist based sensors can be a bit flaky in some conditions in which case an ECG chest strap is necessary.