There is no 'litmus' test for those who may or may not be good candidates for a high tech communication device.  If your child doesn't speak, can't speak, or is somewhere in between always consider an augmentative communication evaluation.  This evaluation can determine what can help, how to get it, and what to do with it when you get it!  I've seen countless children labelled as a 'bad candidate for a device' because of misconceptions about the child's abilities or due to a lack of knowledge about devices.  In reality, those 'bad candidates' may be a communicator waiting in the shadows!  You NEVER know till you TRY!