If you get a dialog saying the object is already checked out by another user you can mark that line and a button emerges to take over the object. However we have experienced a few times that the first user to check it out claims to have lost changes, while the new user takes control of the object.
There is another way to do it. Go to the DB and locate the table SYSVERSIONCONTROLMORPHXLOCKTABLE. It holds all the locks and you can simply replace the userId in the CREATEDBY column for the object which you wish to take over the control for.