You can buy a bit measure from a saddlers, they are usually black plastic straight bar that has solid round ends, you place the straight bar in the mouth like a normal bit they slide the ends up to the side of the lips, slide it out and read the measurement. If you don't want to buy a measure then just put a spare bit in and basically you should have a gap at each side of the lip enough for you to place your thumb on the bar without it pinching against your thumb & lip.
