There are a few stats in service mode but they won't tell anything about the battery health. Just charge your watch fully and keep track of the discharge speed. If it deplates faster than in the past, it's basically lost some of its life.
only with a usb meter, which you can plug between the charger and the watch. otherwise i know from my older garmin watches. if you fully charge them pu and they will not get to 100% for example only to 95% (or as soon as you unplug them) you know, that the battery don't have the full capacity anymore. otherwise, the battery should have the rated capacity.
