1) enter find mode. Put just one character: = in the field and perform the find. The exception to this is when the field is from a related table instead of the layout's specified data source table. Then a modification of 2 below should be used.
2) Enter find mode. Put an * in the field. Perform the find.
To find records in table A that have no related records in Table B, enter Find Mode, put an * in a field from Table B that is never empty of data and click the Omit button so that you are omitting all records that have a related table with some kind of data in that field. Perform the Find.
This can also be done in scripts: Scripted Find Examples
Thanks so much, Phil. That is fantastic. Just what I needed.