Hi Renold, For a PC, inorder to check the clock speed go to the system properties which comes after right clicking my computer and clicking general. There the processor will be displayed and with the clock speed mentioned in GHz (eg:1.5 GHz) .
In Windows 8 and Windows 10: Right click on start and click system
I am not sure about mobile phones but there is an app called the cpu-z which gives all information about the processor including clock speed
Note: GHz determines the clock speed