There are a couple of ways to verify if your data is synced to Day One Sync. 

Server Dashboard

Sign in to http://dayone.me/user/login. If you use Apple ID, click the link to Authenticate with Apple ID (below the Sign in button). 

Here you can view the quantity of synced entries and photos. You can also click Account at the top to manage your sync account. 

Sign out when you are finished.

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