Ideally, garlic plants should go through a two-week dry period before harvesting. In my experience, rain/watering much during those last two weeks not only result in smaller blulbs, but those cloves continue to expand during the two-week "curing" period and will split the capsule/casing. There's nothing you can do about rain but you can control watering during those last couple of weeks.
If you have hardneck, the leaf stems will remain stiff and upright, and you have to go by the browning of the lower leaves. When half of the lower leaves have browned, that is the time to begin harvest. With softnecks and Creoles, the stems are soft and will literally begin to fall over near the ground level, and that's a definite sign that they need to be dug up. So you can use both visual methods to determine the time to harvest.
