It's probably easier to just view the folders side by side on your computer.
Within Lightroom (without plugins)
You could also select both folders (DNG and JPG) by holding down [CTRL/COMMAND] as you click each folder. This will show all the images from both folders together in the Grid view, and sort by filename. (Hopefully you keep the same file-name for both file types!)
This will allow a visual check of duplicates.
You could also narrow the Grid view to two columns (by thumbnail size) to help visualize image pairs.