Getting CPU clock signal out of computer (as to measure externally)
The internal core frequency is generated from an external 133.3MHz BCLK_DP/BCLK_DN differential clock input (using on-chip PLL to multiply the clock frequency). See, for example, this Intel document. 

You won't directly find the very high core frequency signals on the PCB. 

This is true of even very small (relatively speaking) microprocessors and microcontrollers these days- a relatively low frequency external clock drives an on-chip PLL to multiply the clock up for on-chip use. For example, the ARM chip I'm using right now has a 12MHz external crystal but operates at 300MHz on chip.