There are a couple of ways to verify if your data is synced to Day One Sync. This is the Server Dashboard 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.
You can also click your name at the top right, then click Settings to manage your sync account.
Sign out when you are finished.