Some use a switch off the clutch, some will put a micro on the shift lever so when your in first it's engaged. Either works.

I for a while just used used the line lock button on the shifter but the problem with that was I'd inadvertently hit it while shifting and pretty much turns on a rev limiter for whatever pill you have in your two step.

Here's a common clutch switch.
http://www.jerrybickel.com/electrical-components/clutch-operated-switch-jbrc5509.html