The HyperCard 1.2.5 "Find" command is documented on pages 106 and 107 of the "HyperCard Script Language Guide: The HyperTalk Language". Using the "Find" command to search on a specific field works only for background fields. If you do a search on a card field, the string will be found in all fields with the text that matches the search criterion.
The find command is more flexible and powerful in HyperCard 2.0.