1) Go to Manual mode and disble Auto ISO
2) Attach the kit lens and set it at F/22, for example
3) Detach the kit lens and attach a manual lens (or leave the camera
4) Take a picture, and compare its exposure with what you saw on the LCD.
Compilation of possible workarounds (I've just found the one for 60D,
others were known):
Workaround for all lenses, all cameras
* Take pictures in Movie mode
* Use Auto ISO
Workaround for unchipped lenses (60D):
* After using a Canon lens, take the battery out before (or after)
attaching a manual lens.
Workaround for unchipped lenses (550D):
* Detach the Canon lens while camera is on.
Workaround for chipped manual lenses (60D, 550D)
* Simply try to change the aperture back and forth (it won't change,
but the brightness in LiveView will come back to normal)