Ambient Light Sensor: The simplest way to check this is by sitting in a darkened room and turning it on and off in Settings > Brightness. In a dark room and at the default brightness settings, the display should brighten with Auto-Brightness turned off, then darken slightly with Auto-Brightness on.

CPU/GPU: Playing any graphically-intense 3D game will give the iPhone 4's CPU and GPU a decent workout, particularly if the app is optimized for the Retina Display. The iPhone 4's processor and graphics hardware are surprisingly robust for a device its size, so you shouldn't see much graphical slowdown in any App Store game you fire up. Garbled or distorted graphics could indicate a fault in the GPU, while excessive heating or general slow performance could mean a faulty CPU. Playing a hi-res h.264 movie in the iPod app is a good check, too -- videos should play smoothly, with no stuttering or obvious artifacts like blocks, lines, pixellation, or distortion.

Magnetometer: Open the Compass app and tap the button in the lower right corner. Your iPhone should be able to switch between true north and magnetic north seamlessly. If your iPhone is unable to acquire a signal even after you do numerous "figure 8" movements to clear interference, the magnetometer may be faulty.

Gyroscope: The iPhone 4 doesn't come with any default apps that take advantage of its gyroscope. Fortunately, there's a free app called Gyroscope in the App Store that demonstrates the iPhone 4's ability to detect its orientation in 3D space. Once you download and open this app, it'll be obvious right away if your iPhone 4's gyroscope is faulty, because one or more of the values for pitch, roll, or yaw won't change when you move the device.

Things to test during a phone call:

Microphone: Test the microphone in both normal and speakerphone mode. The person on the other phone should be able to hear you clearly, with minimal distortion, if any. There's been a few reports of failed microphones in the iPhone 4, with people unable to hear speech from callers using an iPhone 4; whether this is a microphone fault or due to an overzealous noise-cancelling microphone is unclear.

Speakers: Both the earpiece speaker and the bottom speaker should be audible and free of static or distortion. Listen for interference from the noise-cancelling mic or an unshielded/poorly shielded earpiece speaker -- my first two iPhone 4 units made a very annoying "beatbox" sound during phone calls.

Proximity Sensor: The touchscreen should shut off when the iPhone is held up to your ear, then swiftly come back on when moved away. There have been reports of iPhone 4 proximity sensors randomly activating during phone calls and causing people to inadvertently hang up on calls with their cheeks (how's that for FaceTime), but this is likely a problem with the iOS software rather than a hardware issue. If the proximity sensor doesn't work at all, that's a hardware fault.

All of the above tests done together take about 15-20 minutes, not counting the time it takes to sync data onto the iPhone for the first time. Putting the iPhone 4 through its paces using the tests above should find any glaring defects with the iPhone 4 right away. The last bit of the iPhone 4's hardware that requires testing takes significantly longer to test:

Battery: I haven't done any specific tests of the iPhone 4's battery, but the life I've gotten out of it over the past 24 hours is consistent with Apple's estimates. There's any number of battery tests you can perform to test your battery's capacity, but one of the most popular is putting a video on repeat and letting it play until the battery runs dry. According to Apple's tests you should get around 10 hours of video playback out of one battery charge, but there's some leeway in those results. I'd say that anything less than 7 hours or so of video playback on a fully-charged, brand-new iPhone 4, at default brightness, that's done nothing but play video during that time, may indicate a battery fault.

These tests are necessarily simplistic, and they may not catch all the hardware faults that an Apple Store Genius might find with specialized equipment. That's the beauty part, though: these are tests just about anyone can do, and if you do them all right after getting your iPhone 4, you'll be relatively sure your iPhone 4 isn't an out-of-the-box dud. After running through this list myself, I couldn't find anything wrong with my second replacement iPhone 4, so for now at least, it appears the third time is indeed the charm.

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