Logical validation is a process purely intended for testing the functionality of a chip, and it involves three types of tests. The first category comprises benchmarks, games, operating systems, server applications, and so on. This is the easiest category to achieve success in, because this sort of 'normal' software usually does not contain very exotic code.
The second type of tests is harder, and involves trying out specific (new) features. Usually, these tests are written by the chip developers themselves, since they know best how to cover every possible peculiarity and exception for their design.
The final category is probably the hardest. It involves firing random instructions and data at the processor to see if the physical chip behaves in the same way as the software model of it. Naturally, the 'emulator' is tested extensively before the first chip arrives from the factory, so if the hardware implementation responds identically to every combination of instructions and data, then chances are that everything is fine.
As can be expected from one of the largest technology companies in the world, nearly everything is done automatically. A large network takes in tests and returns the results automatically, so the employees only need to concern themselves with the actual problems.
When a problem is found, the main job for the people in the lab is to figure out what is causing it, since it does not necessarily have to have anything to do with the processor: software, bios and chipset can cause crashes too. By attaching the test platform to a whole range of hardware, the flow of signals across the buses and the state of the processor (such as the contents of the registers) can be determined with great precision, so that a diagnosis can be made. Sometimes the real problem is in the testing software, rather than in the actual product. All in all, finding a cause can be quite a puzzle.