How do you know when a device needs reballing?

Jaywn, reballing is usually performed to replace a BGA IC, not a device. It has become somewhat more accepted to reflow devices like PS3 XBox etc. Most of the time a reflow is based on generally known failures. It is the thermal cycling that causes the following in Lead-Free Solder over time.

Weak bonds

Solder Voids

Organic Matter Build Up

Cracking Brittle Joints

"Black pad" caused by the formation of brittle intermetallic layers Pad cratering, It is the space left vacant when a BGA ball pad lifts off the board. All of these are caused by the solder going through so many thermal cycles due to the lack of heat dispersion, simply put the solder spheres are too old and have been through too much. "The affects of thermal cycling on BGA packages have increased since regulations were put into place concerning consumer electronic and the use of Leaded solder. This meant that all consumer electronic items must only use Lead-Free solder which is much more brittle and affected by thermal cycling. Therefore replacing the solder is the best route, also to reduce the damage each thermal cycle causes we have fabricated a heat sink modification to reduce the run heat of the GPU by 30%. Not all BGA Package IC’s can be Reballed, this is due to the size and thickness of the BGA IC’s. " To reball an individual IC you will know the pattern of the array, the size of the solder balls. Then you will need a stencil, replacement solder balls and a reflow station. You will also need the proper temperature guide to ensure a proper reball. I consider this a bit of an art. It does require lots of practice, knowledge and the right tools to reg it accomplished. Hope this helps, good luck