The boot rom should print out the CPU/FPU speed on the display.
You may also be able to redirect the console to the serial line.
This is done either by a switch on the IO board (older CPUs) or via
the boot rom (newer CPUs).  You want to put the rs232 interface in
"remote" mode.  That will override the display.  Or you can pull out
the display card (if that is possible).