As some of you know from my post not too long ago, I recently started using a Lee Loadmaster for 9mm and .45. So far so good, but I have not liked the "blind" primer seating - especially after reading on the interwebs about upside down, cockeyed or missing primers. The small LED light under the shell plate idea is great for verifying that a primer is there but does nothing for an upside down or skewed one.
I recently bought a usb endoscope for work and thought this might work for checking primers as well. You can get one for less than six dollars on your favorite auction site. I splurged and spent eight bucks for one with an adjustable light intensity. One thing it must have is the 90 degree end adapter. There are several free OTG apps that allow it to run on your phone. I drilled a hole under the shell plate and epoxied the end once I got the location right.
The photos aren't the best, but you can see the results (I'm still playing with the light and contrast). The images do look better on my phone's screen.
Here's how it looks with a good primer in a case.
Case without primer
No case in the shell plate
Not the best image quality but it does give me a visual confirmation and a little peace of mind.