You can guess why I open this issue. It happened to me that I really scratched my head trying to figure out which passphrase I used for a key and then I noticed Caps lock was on........
Windows, KDE Plasma and other apps have a warning when capslock is on.
Would be another string though that is passed from gpg-agent to pinentry.
For Qt:
https://stackoverflow.com/questions/2968336/qt-password-field-warn-about-caps-lock