Just ask it the definition of a mathematical term and watch it get it wrong. For example, I asked it to define the Kronecker product and it gave me a lot of correct information, but the actual equations were wrong.
This has always been the argument for technologies that failed too. In many cases it turns out that it is rather "easy" to get the first 90% working but the remanining 10% are not only exponentially more difficult to get going, but also what is the difference between a tool and a toy.