Is it time, already? Actually the hot weather has pushed harvest of these root crops a little earlier than normal this year, and some garlic growers actually started harvesting a few weeks ago (very unusual).
Typically we know the garlic is ready to harvest after the leaves turn from green to yellowy-tan, and start to whither down. Once we see these clues it is worth digging a couple of sample bulbs, which should look full sized, plump and the skins should be intact. If left too long the wrapper will start to become really papery, and the garlic bulbs will not store well - so it is worth erring on the side of caution to avoid this predicament.
Aside from timing there a few extra notes to follow when it comes to garlic harvest. First, dig gently as any scuffs to the bulbs will compromise storage and potentially lead to rot. Once out of the ground, keep the tops on but trim away 1/3 of the roots and tap off any loose soil. Second, garlic needs to be cured in an airy location for several weeks before storage; once cured the tops can be trimmed-off (the hardneck garlics that thrive in our region cannot be braided!).
There is less of a single season for potato harvest, since there are early, mid and late season types. The earliest potatoes have been coming out of the ground since mid-July, but regardless of type... and even if you have forgotten what you are growing... you can confidently dig once the flowers have died back.
Similar rules of thumb apply in terms of digging gently to avoid damage to the skins, which reduces storage for potatoes in the same as it does for garlic. Avoid anything other than a light brushing off of dirt until the potatoes have cured. Potatoes should be cured for about two weeks by laying them out on cardboard or newsprint in a cool, dark and well ventilated location; this makes the skins tougher and significantly improves storage ability.
Many people ask about the Ketchup-N-Fries plants at this time of the year. The plants put out tons of the sweetest-ever cherry tomatoes on strong indeterminate vines, but when to dig the potatoes? Since there is no potato flowers to clue us in the rule if thumb here is to simply wait until cool weather stops the tomato production, then dig the potatoes.