Is there a way to tell which files I accidentally renamed to *.jpg aren't actually JPEG images?

You can use file FILENAME to determine the type of data in a file.

$ file image.jpg text.jpg pdf.jpg
image.jpg: JPEG image data
text.jpg:  ASCII text
pdf.jpg:   PDF document, version 1.4

So, file *.jpg | grep -v JPEG should give a list of all non-JPEGs with the ".jpg" extension

Also, assuming there are PDFs you can rename them in one go

find . -type f \
    -exec sh -c 'file "$1" | grep -q PDF' _ {} \;  \
    -exec sh -c 'echo mv -- "$1" "${1%%jpg}pdf"' _ {} \;

Remove the echo in the second line once you verified that the output looks ok.

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