Oh not that surprised. I just didn't realize is all, my bad!
Unfortunately, the best you've got is in the watch app > general > usage. At the very bottom, you'll see some battery statistics, but they aren't of much help.
That said, all batteries fail eventually, and all batteries lose charge gradually over time. Anyone who has had an S0 watch and has used it regularly since its release will almost certainly see an increase in overall capacity by getting its battery swapped. If you charge the watch every day, you're probably pretty close to if not already past the 1000 cycles that the battery is rated for. Even if you're at 600-800, you're well on your way and a new battery will certainly help.