istocknow.com but right now they aren't really reliable to use because it's been broken since launch day.
Otherwise you'll just need to check on apple.com, select your model that you want to get and check to see if it's available for pick up in store. Every 6 AM, it updates the status, every 6:30 pm, they disable the status. Apple does receive shipment on a daily basis but they don't typically know what's in the box and once they open them they get added to their inventory.