Callouts have nothing to do with forms per se. Forms are in the client,
callouts run on the server.
The object ID of the entity being changed is passed to the callout, so you
can always do a retrieve using that object ID to get all the attributes
(fields) of the entity.
You can also pass before and after values of those attributes (depending on
the type of callout) to the callout by using pre and post value tags in the