Knowing how and when to harvest potatoes from your garden can be a tricky proposition. After all, the part of the plant you're anticipating baking, boiling, or mashing is underground, covered most likely with soil and maybe straw (or whatever else you use to hill your potatoes with) as well.
Luckily, there are a couple of very simple ways to tell when it's time to harvest your potatoes. Depending on whether you want "new" potatoes or potatoes that are good for storing, there are a couple of things you'll want to look for.
If you are looking to harvest tender new potatoes to be consumed right away (not for storing) then you want to keep an eye out for the blooms. Once the plants start to bloom, you can start gently digging for new potatoes. These should be washed and cooked immediately, as they don't store well at all.
If you are planning on harvesting potatoes to store for a while, wait until the tops of the plants start to yellow and die back. Then gently dig around the perimeter of the plant and dig up the tubers. If you are planning on storing them, don't wash them! Let them sit out in a cool place for a few days to cure, then gently rub off any dirt, and store in a cool, dark place.