There are a couple of ways to verify if your data is synced to Day One Sync.
In the settings/preferences, the sync status will show when sync was last updated.
iOS - go to Day One > Settings > Sync Status
macOS - go to Day One > Preferences > Sync. There is also a sync status icon below the list of journals in the journal sidebar on Mac.
Android - go to Day One > Settings > Sync
The Server Dashboard is accessed via web browser. It is used for verifying synced data amounts and for account management, including changing email/password.
Sign in to https://dayone.app. There are multiple ways to sign in, depending on the method you use in the app (Apple ID or Day One account- email and password).
Here you can view the quantity of synced entries and photos. Compare this information with what you see in the app.
You can also click your name at the top right, then click Settings to manage your sync account.
Sign out when you are finished.