There isn’t a widely recommended free desktop tool mentioned here that specializes only in finding panorama candidates, but there are two sensible ways such software can work:

1. Analyze image content to detect overlap between frames.
2. Use metadata such as capture time and similar shooting settings to group likely sequences.

In practice, metadata-based detection is often much simpler and can give nearly as good results, especially if you shoot panoramas as a quick sequence with locked exposure/focus.

One free example mentioned is Google+ Auto Awesome, which automatically detected panorama sequences and created panoramas for you. It worked by recognizing related image sets, though it was more of an automated cloud feature than a dedicated desktop workflow tool.

So the short answer is: yes, free automatic panorama detection has existed, but dedicated standalone freeware/open-source tools for detection-only are not clearly identified here. If you want to build a workflow, grouping by timestamp and similar EXIF settings is likely the most practical approach.