What year is it? the Haynes manual is only any use if you read chap13, suppliment.
The speed transducer produces pules via a reed switch, this is fed via a buffer to the EAS, Engine ECU and the speedo. If it was knacked then the ECU might show a warning, but you can check it by lifting a wheel at the back, turning it slowly and checking for a pulse at the output of the transducer/input of the ECU, it tells you how to check this in the suppliment under the ECU diagnostic, which is very good BTW.
The speedo heads do have something of a reputation for failing though, especially the oddometer.