You can use C. Fn. IV-07 to activate Simulate Exposure, so that the LCD won’t maintain a constant brightness level under varying ambient lighting conditions but will instead brighten or dim to emulate the correct exposure or over-/underexposure you’ll get with the current settings.
Enabling this feature also activates the histogram, which you can use to judge exposure (as explained in Chapter 4). If the histogram is not visible on the Live View screen, press the Info. button until it appears. The histogram may not display properly under very low or very high light levels, and is not available at all when you’re using flash or exposures with the Bulb setting.