onions and garlic

when will i know they are ready to harvest.....
i have lost so much already this year through waiting till i think it is time and then my pak choi and chinese cabbage going to seed

 

when the leaves flop over with them both. My onions are ready now, they are japanese so i planted them in Oct/Nov last year. So what I should do is dig them up carefully and leave them on top of the soil for the skins to form and dry for about two weeks then store them in net bags in the garage. that's what I do anyway.. My garlic leaves are not drooping yet so I presume they are not ready.