Use the APIs that Apple provides:
if ([CMStepCounter isStepCountingAvailable]) {
// The device supports step counting
} else {
// The device does not support step counting
}
if ([CMMotionActivityManager isActivityAvailable]) {
// You can use CMMotionActivity
} else {
// Nope, not supported
}
Of course this API is only on iOS 7 or later. So if you need to support iOS 5 or 6 then you need to wrap this code in a check for the CMStepCounter
class too.