Go to the tags browser (the pane on the left). Click the luggage tag if you need to open it.
Click the triangle to expose the formats. Change the "match any" box to "match all". Click RAR until it shows a green +. Hold down the shift key and click the others until they show a red -. This will show you books with RAR and nothing else. Save it as a search to simplify your life next time.
If you want to play a bit, use a regular expression in the search. For example
Code:
formats:=TXT and not formats:"~(EPUB|PDF|RAR)"
will find all TXT formats that are not also EPUB, PDF, or RAR.