When is Apple Watch backed up to a paired iPhone?
You can be sure the watch backs up only when you unpair it. 


https://support.apple.com/en-us/HT204518


Apple says it backs up automatically but since you can’t check on that by tapping anywhere or logging into iCloud or looking in preferences, you don’t know when it happens or the date of the last backup. 

The only way to check your available backups is when you pair any watch (e.g. you have a second watch or a friend that’s willing to unpair theirs or has a spare) and step through the pairing process and have your iPhone list the available backups and date ranges of backups. iCloud keeps multiple backups in time, sort of like time machine so you may have a choice of when to restore from your backups.