Remember back when there wasn't a radar gun and we just had to guess how fast pitchers threw? Yeah, I don't either, but apparently it was like that once upon a time. I can't vouch, so we'll just have to trust this excellent YouTube video of Hall of Famer Bob Feller having his legendary heater measured by Army ordnance equipment:
As the video says, Feller's fastball was measured at 98.6 mph in the above footage. Obviously we can't accurately estimate how hard the Indians ace routinely threw during games, but I'd bet he could have hit triple digits at least once if he was throwing 98.6 through a machine in non-game action. I'd also bet he generally sat lower on a regular basis -- like 95-96 -- when he wasn't trying to impress, via a machine, and was instead trying to work deeper into a game.
But that's all guesswork and we probably don't need it. Feller was a truly legendary strikeout artist, leading the majors seven times (1938-41, 1946-48). The eight-time All-Star would end with 2,581 strikeouts in his career.
And one time he actually threw some pitches through Army machinery to have his fastball velocity measured. That's awesome.