A jerk is basically the slew-rate of acceleration, so a very easy way to get the jerk value is by mounting an appropriate MEMS accelerometers chip to the print-head (and or print-bed, depending on what is moving). Getting the jerk value is then a matter of reading it out (over I2C, SPI or whatever) and then discretely differentiating the acceleration values. When measuring position instead of acceleration (e.g. with encoder discs) one needs to differentiate trice