Field information is shown on each control in the top of the field attributes pane:

 

 

Field Information is useful where a control is used on more than one page as a "Used Control".  Clicking on the Field Information button will display clickable links to the pages on which the control is used within the current script, and a list of all reports which use that control:

 

 

 

Where a field is used in a report, and is subsequently deleted, the column will still appear in the report, but with no data.  It will appear in RED in the field list for the reports it is used in.