For Gtk+, set the GDK_BACKEND environment variable. Open a terminal and execute GDK_BACKEND=X11 gedit for example.
For Qt, the default (in Fedora 28, as of 2018-07-29 and earlier) is to start any application with X11 backend. This may change in the future. the -platform command line option may help. Right now, you can enforce using the wayland backend by executing a Qt5 application like this: kwrite -platform wayland. There is probably an option for X11 too.