1. While on debugg mode follow the menu path GOTO->System->System Areas
2. A list appears on the debugg screen
3. By default the area editbox has a ?
4. Now enter DSEG into that field.
5. Press return key.
6. You will get a list of all the tables active in that program with some other attributes.
7. The type and size of the records determine if they are internal tables and how many records there are.