When a client submits response to an album, you will receive their response via email, and it will be logged as feedback in Backlight's admin.
To locate received feedback in Backlight's admin, go Client Response => Feedback.
The response will contain one or more lists of comma-separated file names. Assuming you have been following file naming best practices, this makes it very easy to isolate the respective images in your library.
As an example, you will have a list like this one:
Copy-and-paste the list of file names into your library's filters to isolate them for further processing. Mark the images with a color, flag, keyword or star rating, or create a new collection containing only the filtered images.
Filtering Images in Lightroom
Copy and paste the list of file names into Lightroom's Text filter, with the settings:
Text | Filename | Contains
The images will be isolated in your Library.
The filter will be applied to the currently selected folder or collection, so make sure you're in the right place, or just select All Photographs under Catalog on the left side of the Library.