Azz .. Tom wouldn't have been able to tell from just looking at the fish how many times it's spawned. Scales must be examined (with the use of a microscope) to count the annuli and from there a life history can be gathered.

Perhaps you misunderstood what he meant ... sounds like he likely figured it was a 3 salt fish, meaning that it had spent three years at sea, and would likely be an educated guess from the size of the fish and a little experience at checking