@mq ok nvm i found out how to check it
if (mc.gameSettings.keyBindSneak.pressed == true) { /* If crouching */
mc.thePlayer.motionY = -0.31; /* Go down */
} else if (mc.gameSettings.keyBindJump.pressed == true) { /* If jumping */
mc.thePlayer.motionY = 0.31; /* Go up */
} else {
mc.thePlayer.motionY = 0;
}