If You are using KDE, go to the KDE Control Center and search for "keyboard shortcuts"
In the Keyboard Shortcuts dialog, you can look up the function you
want, and it will tell you if a key is set, what it is and you can
change it if you want.
There is also the application xlock, and you can call that from the
KDE command dialog, or create a .desktop file and bind a key to it to start