Go to your BIOS look in your motherboard specific BIOS for the overclocking section and see if the CPU is set higher than advertised in the add.
If you mean to check whether it has been overclocked in the past. I am not sure that is possible. If they clocked it back down before listing and selling it I doubt you can check for it.
To be honest as long as the hardware is less than 5 years old I wouldn't worry about it. Most people now a days don't overclock like idiots they just push a 2,9 to 3,2 or a 3,4 to a 3,9 ect. they don't take them to outrageous clock speeds.