Green’s Dictionary of Slang

(orig. US) one who collects information on, and forecasts the result of, sporting events, elections etc.

1904		St Louis Republican (MO) 28 Aug. 21: ‘Information Men’ at union Station [...] the Young ‘Dopesters’ of the Bureau in the World’s largest Railway depot Untangle Knotty Problems for the nervous Traveling Public.
1906		L.A. Herald 27 Apr. 6/4: They are matched to box in Los Angelees for $20,000 - a wonderful conclusion of the eastern dopester.
1917		Sun (NY) 1 July 11/8: What you boys want to do [...] is to change doopesters.
1920		Ade Hand-made Fables 174: Only a real Dopester would have given the Tip that Claude carried all the Weight and that Silas was an Odds-On Favourite.
1933		‘Paul Cain’ Fast One (1936) 146: Gilroy was [...] a heavyweight who had been at the top of his class [...] The dopesters said he’d lost everything he ever had.
1942		A.J. Liebling ‘The Boy in the Pistachio Shirt’ in Telephone Booth Indian (2004) 214: Some of the dopesters he listened to [...] have a high opinion of German prowess.
1948		News-Dispatch 3 Apr. 9/3: It was a big upset according to ringside dopesters [DA].
1962		Ragen & Finston World’s Toughest Prison 797: dopester – One who acquires information. [...] One who offers information on horse races.