There are four Form Response statuses that will be sent back to Salesforce when a user has started or finished a Smart Form. This is available in the Form Response's Status field. 

Form Response Status Definitions

Response StatusDefinition 
Draft The form has been started but is not yet complete, either because it is in-progress or the mobile user has exited the form but has chosen to save and continue the form later.
AbandonedThe form was started but the user exited the form before completion and chose not to save progress. Everything completed up until the user chose to abandon the form will be sent in the Form Response back to Salesforce.
Completed The user has completed the form but there are still mapping/file updates that are pending.
Completed-SyncedThe user completed the form and all updates have been synced to Salesforce.

Example Status on a Form Response Record