How can the Ames test detect a human carcinogen?
The Ames test aims to find out if a chemical (not yet known to be mutagen) is indeed a mutagen, either directly, or following treatment with liver enzymes (which may metabolise a chemical to mutagenic derivatives during "detoxification"). As you explain, the test involves looking for reversion of various histidine auxotrophic strains of Salmonella.

If the chemical is found to mutagenic in the Ames test then there is a good chance that it will be a carcinogen because of the link between mutagenesis and cancer. In practice further unrelated tests are needed to eliminate false positives.

In this review of the method


  Mortelmans &amp; Zeiger (2000) The Ames Salmonella/microsome mutagenicity assay. Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis. 455: 29-60


the authors state that:


  ...there is a high predictivity of a positive mutagenic response in the Ames test for rodent carcinogenicity...ranging from 90% to 77%...