How to Tell: you'll see a flashing folder icon and a question mark, a spinning coloured disc for too long, or a completely white screen
Batteries swell up and warp case.
Funny noises - high pitched tones, clicking, moaning/mooing sounds.
Keyboard / trackpad stops working, or 'loses' letters when typing fast.
LCD has uneven brightness / dark corners.
LCD has pink lines on the display, scrambled display or distorted graphics.
Random shutdowns, shutting down when unplugged (even when battery newly charged).
Drops WiFi signal.
Liquid spills - either hot or cold drinks.
Accidental knocks, or dropping the machine.