Is there any easy way of finding out which images LR is not showing
Easy way? Depends on what you mean by "easy".
One possibility is to synchronize the folder, this will add (import) the photos that are not previously imported into LR. CAUTION: this has a disadvantage that it will also remove photos from Lightroom if they are not in that folder. It also will fail if your photos have previously been imported into LR. Repeat as necessary.
Another possibility: You could, in Lightroom, drag the photos to a temporary folder, then look in your operating system to see what photos have not moved, those are the ones that are not in Lightroom. Then, in Lightroom, drag the photos back to the desired folder. Repeat for each folder that is of interest.
Side comment: you might want to rethink your workflow if you are winding up with some photos in a folder not imported into LR. In my mind, you import all photos (that's every single one, 100%, no exceptions) in a folder into LR, and you do this all at once so you don't wind up with the problem you are describing.
Synchronising did not find any missing photos. However, what I have done since posting the above is: I copied the entire folder to another drive and renamed the original folder so LR put a question mark against it; then I selected 'Find missing folder' and directed LR to the drive where I had copied the folder; and voila, all of the photos were back!
On reflection, I think what might have happened is that I may have told LR to remove these photos from the catalog; following from which the question is, how does one put back in the catalog photos one has asked LR to remove (and synchronising doesn't seem to do that).
I do incidentally import all the photos in a folder into LR at one go, so that doesn't account for the problem here.
Synchronising did not find any missing photos.
This usually indicates that all photos were imported into the LR catalog, but some of them were imported using the COPY option and Lightroom thinks they are in a different folder. However, note I said "usually", if you are sure that you import all photos in a folder in one go, then my first sentence does not explain why Synchronizing finds nothing. Is there any chance that on one or more occasions you only imported part of a folder?
Another possibility is that instead of deleting the photos from the LR catalog, you moved them inside the LR catalog to a different folder. Is there any chance you did this? You may have done it accidentally and not even realized it — we all click in the wrong place sometimes.
If the photos have truly been removed from the LR catalog, then importing or synchronizing should work without issues, and since it doesn't work, I assume that the photos are still in the catalog, but Lightroom thinks they are in a different folder.