I do not know anything about detecting artificially toned coins. However, neither dealer suggested that the toning was artificial. In fact, the PCGS dealer said that he believed tht the toning was natural.
Welcome to the forum and congrats on a nice 1917 Lincoln you have there. Well, both top grading services (PCGS and NGC) certainly grade toned coins, and you will sometimes see toned coins in NGC holders with stars on them to indicated superior eye appeal. I can only imagine that the dealer possibly was referring to the fact that toned Lincolns have not caught on the way that toned silver coins have in the marketplace as far as desirability. Also, with the recent problems with blatant AT in the market, the grading services have been somewhat tougher slabbing toned coins. I know of a number of natural toned coins that have been in cardboard albums for years that have come back with "questionable toning," as it can be difficult to differentiate between artificial toning and natural toning. However, I feel that either PCGS or NGC would certify this coin, unless there is some PVC remnants on the coin that Zane mentioned.