To see the shutter speed in Av mode, compose the shot and press the shutter button halfway. The shutter speed will show up in the viewfinder alongside the aperture you have set, in the same spot both numbers always display. Because you've pressed the shutter only halfway, the camera will not take the picture.

Keep in mind the camera is determining the appropriate shutter speed based not only on the aperture you have locked in, but also on variables such as the ISO you have set (or any auto-ISO settings you have configured), the available light in the scene, and the light-metering mode (evaluative, center-weighted average, etc.) that you have chosen.

The camera takes these settings into account and attempts to choose a shutter speed that produces an even exposure. Watch the -3...+3 bar in the viewfinder. In an even exposure, there will be a vertical line underneath the midway point of this line. A vertical line to the left of this point indicates the scene will be underexposed, while a vertical like to the right indicates the scene will be overexposed. (You may be okay with under- or over-exposures based on what it is you are shooting.)

Follow the 'press-the-shutter halfway' tip to see the aperture while in Tv mode. Everything above is still true except replace the word 'aperture' with 'shutter speed' :-)

To see both shutter speed and aperture without pressing the shutter, switch to the P or M mode. However, in these modes, the camera doesn't set either number automatically. You change them yourself based on the kind of picture you want to take.