there should be a self diagnosis.
switch on and idle engine. the rev counter needle should alternate between engine speed an 4000RPM if there is a speed sensor problem detected electronically
if the electrics are fine and no speed is showing when riding then the issue is more likely to be a duff speedometer or clogged up sensor
to test the sensor you will need to remove it and uncouple it at the harness. connect a 12v supply, +ve to the blue/yellow sensor side terminal, -ve to the black sensor side terminal. connect your meter set for DC VOLTS , COM to black sensor side terminal, RED to white sensor side terminal. slowly SLOWLY turn the spindle in the sensor. The volts should alternate between ZERO, and two volt readings - three sets of readings all together. the readings will be like 0V, 1V and 12V alternating. If you dont see this then the sensor is knackered.
EDIT: Make sure you disconnect the bike battery when doing this !! use it for this test by all means but do not have it operating the bike electrics