Does it appear that your onions and potatoes are dying?  Maybe garlic too?  Or did they seem to die a few weeks ago?

When potatoes, onions, and garlic begin dying (especially if it is mid to late summer or later) they may just be ready to harvest.  Try pulling the soil away slightly or digging them up.  If the onions and garlic have formed bulbs and the potatoes have tubers then go ahead and harvest what you have.