If the master puts tefillin on his slave or helps him marry a free Jewish woman, we know that he had previously freed him.

A slave of two masters was freed by one. The second transferred ownership to his underage son, to prevent the court from emancipating the slave. The court commanded to jingle a few coins in front of the child, who agreed to sell the slave and was then paid in full.

If one says, "I make my slave a free man," the slave is free. But it has to be in writing. Same for a field transfer.