|I was making my daughter's uniform shorts (Kwik Sew 2779) for school and ran out of the ribbon I usually use to mark the back. Navy shorts plus sleepy and in a hurry = shorts sometimes put on backwards.
Before attaching the elastic, machine embroider or narrow zig-zag an initial in a contrast thread color within the casing allowance area. Or use one of your machine's decorative stitches.
You could also make a tiny thin zig-zag stitch line near the fold if you've already completed the elastic attachment. If the bobbin thread matches the fabric, this won't be noticeable on the right side of the shorts.