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The easiest way is to simply go to Windows Start and select Recent Items. Unless the person thought to erase recent items, you should be able to see if anyone was poking around. You can also check the recent activity on any of your browsers to see if websites that you have not accessed are in there.
A more certain way is to check Windows Logs. Go to Control Panel>Admin Tool>Computer Management, then click on “Windows Logs” on the left side of the window. Choose the Security log and you can see a log of all log-on, log-off events (among other things), and you can check for activity during those times you are certain that you were not on the machine.