How can i use Sync API to check if a file is deleted by other computer?

We are evaluating Sync API for our file sync function in a new app.
Our app stores user data in local file system and allow them to sync the local file system with a folder in users' dropbox account.
Here comes a question:
When we compare all local files with their remote correspondences. If there is a file exists in local, but not in dropbox,
How can we know if it's deleted from dropbox by other dropbox clients or it's simple a new file from local?