Do you have any idea how many flights there are in the average day? There is no way there are anywhere close to enough Air Marshals to have them present on more than a small portion of the flights.
I had never heard anyone at the TSA claim that there were Air Marshals on a large percentage of the flights. I've heard the opposite many times. I don't know where you might have heard that they had agents on most flights, but they simply don't have the manpower to do that.
I know a way, but it's definitely NOT politically correct.
;-)