Because the cause of GBS is unknow it makes it really hard to diagnose. The syndrome is only recognizable when it has reached out to the whole body. But, there are three test that can confirm it before it reaches those extreme cases. There is the Lumbar Puncture (spinal tap), Electromyogram (EMG), and Nerve Condition Velocity (NCV) test that can help comfirm the diagnoses.
More Detail on these test click here