If you're really, really curious, you can do one of the following:
On XP, right-click the desktop and click "properties." The screen resolution is set in the right-most tab ("Settings", I believe it's called).
On Vista, right-click the desktop and click "personalize." The screen resolution is set in the "screen resolution" applet.
On 7, right-click the desktop and click "Screen Resolution."
Not like it really matters much anymore. LCDs are good at feeding your video card their native resolution, and your video card drivers are good about picking it. The only time you really have to fuss with it is when you're tweaking game settings or setting up a CRT monitor.