If the bike is a 1G B6/B12 or a B4, it has a speedo cable between the front wheel and the speedo head.
If it's a 2G or 3G B6/B6.5/B12, it has an electronic speedo driven by a signal off the countershaft sprocket. The pickup sensor is on the countershaft sprocket cover. IIRC, there's a connector up around the side cover for the sensor's wires, but if not, you could disconnect it by taking the trigger off the countershaft sprocket so it won't sense any speed info.