I have never seen any evidence for Dr Crook originating the spit test, especially as I know that he advocated (and originated?) the candida questionnaire.
Even then I did a search for Dr Crook and Spit test on google and all I got were pages which described Dr Crooks test - i.e. questionnaire, and also another test - the so called spit test. Dr Crook is consitently (and only) associated with the questionnaire. He is never associated with the so-called spit test.
No one has provided any evidence (either scientific, experimental, explanatory or even anecdotal) as to the workings of this so-called spit test.
All the evidence I have seen is that it is a tool used by Multi-level marketing companies to push their products.