The primary way of determining the speed of a computer processor is to look it up in the operating system's documentation. This will vary by whether your machine is governed by Microsoft, Apple, Linux or another OS.
If you have a Microsoft machine, click on the Start Menu, navigate to the Control Panel, and click on System and Maintenance. Go to Performance Information and Tools, which will bring up a chart showing the CPU's clock speed. Apple users will click on the Apple Menu in the top left corner, then select About This Mac. The process is a little more complicated for Linux machines, requiring knowledge of programming. Many computers list out the CPU speed during the boot sequence, although it is displayed briefly. If the speed is not displayed, press the "ESC" key to freeze the screen. Finally, consider downloading an app such as CPU Speed Professional.