I don't believe there is a way to do that. But why are you keeping JPEG images? I shoot raw only. The only time I need a JPEG is if I want to share it on the Internet, via e-mail, or send to a lab to be printed. Other than that, the JPEG's are not needed. So I generate JPEG files for specific purposes. When that purpose has been fulfilled JPEG images are deleted. They can always quickly and easily be regenerated when necessary.
Of course, for those images for which you only have JPEG master files, you will definitely want to keep those. But, in my opinion, there is no need to have JPEG copies of every raw file. Suppose you decide to make additional adjustments to one of those raw files. Then you have to remember to generate another JPEG file, and remember where you are storing them. I just think you are making more work and confusion for yourself. But then, that's just my opinion.
In the past, that was because it was my only way to get photos into Flickr, or add to an email or Facebook post. Plus Windows does not show thumbnails for RAW images so finding photos to add was a problem.
As a new user to Lightroom I am only just beginning to see what it can do. I have noticed that there is a section bottom left that gives options to export to Hard Drive, Facebook or Flickr. your suggestion about not keeping jpg files does seem to make sense if I stick with Lightroom - I am not sure I can justify £8.52 a month on top of my other costs for this hobby.
I have also thought through my processing. I have been using Photools iMatch for cataloguing for many years, because of its hierarchical keywording. The only way to get those keywords into Flickr was to have a jpg that iMatch could propagate them to. Without that I just got the photo in Flickr or Google maps or whatever, without the important keywords. One of the things I had hoped to do in Lightroom was explore its hierarchical keywording (which is fairly new) and see if that can take over from iMatch. Sadly I've missed over 3 weeks out of my 30 day trial due to illness, and Adobe are utterly uninterested in extending the trial.
If you post to those websites from Lightroom, Lightroom will make the conversion and post JPEG copies of the images. As far as Windows is concerned, do a Google search for a raw codec for Windows. You will be able to download a little file and install it. Then Windows will be able to display your raw files. If you don't continue to use Lightroom, you will probably get something different that will be able to support your raw images. After all, you have discovered the value of working with raw. But the choice is yours. If you feel compelled to generate JPEG images for everything you do, then go ahead and do it. There is technically nothing wrong with doing that. You have to do what makes you feel more secure. There just isn't a way using Lightroom to discover which files don't have corresponding JPEG images.
Thanks for trying. You are probably right in not needing jpg files permanently - they only take up space. My camera was launched in late 2013 or early 2014. Windows 10 is SUPPOSED to have inbuilt RAW codecs, and it didn't come out till long after my camera, yet it isn't recognised. One of the threads in Microsoft Community recommends installing the Windows 8.1 codec pack, but that will not install. The only solution appears to be to install a codec from FastPictureViewer, which MAY work, but at another cost.
What I have found is that Adobe Lightroom makes a complete hash of the keywords already created for my files. It adds "<Imported Keyword Tags" to everything, as well as showing the keyword without that.
It all seems like too much trouble...