A pretty quick search would likely yield
xev as a result. It will not show you everything that is pressed or typed in X ever. But rather, will allow you to see information about keycodes and mouse movements.
However, with the
-root option, you might be able to get
xev to monitor the whole X session. Note, if you do this, you'll make it pretty difficult to actually interact with any program since
xev will be capturing all input.
xev -root comes with an implied "YMMV". Another possible solution is to use
xinput to get the job done (though you need a recent version (1.6.1 or above)):
$ xinput test-xi2 --root
While the above should be able to capture all events, if your
xinput doesn't support
--root, you can use
xinput test on a particular keyboard or mouse device.
See also this other question: Record every keystroke and store in a file