Getting the state of the caps lock key is something I’ve implemented myself as a free function, but it would be nice to have out of the box in JUCE as part of the ModifierKeys class.
Here’s the code for it (I don’t have a Linux machine myself, so I know it compiles on Linux CI, but I haven’t tested that implementation myself): https://github.com/benthevining/Lemons/blob/main/modules/GUI/lemons_gui/utilities/CapsLock.cpp