Form definition is by default locked from other users in the designer page. 

Meanwhile the form definition may be opened by another user (with View, Design permissions) and an alert will appear warning the user that it is being edited.

Select Utils - Form Definition Locks menu option to view the lock status across all definitions on the new page.  

The UI is rendered as shown below. Users and the form definitions locked by them are listed in alphabetic order. Select the form and right-click or click on Actions - Unlock menu option to UNLOCK the definition selectively. 

Select the User and right-click or click on Actions - Unlock menu option to UNLOCK all the form definitions. 

Select the form definition and click on View - History menu option. 

The form definition history includes the LOCK and UNLOCK events as shown in the image below.