Great hitters tend to hit the ball really hard. If you wanted to pick a non-cumulative stat to know if a hitter was one of the better hitters in baseball, max exit velocity would be one of the better choices. Here were the leaders in max exit velo in 2021 according to Baseball Savant:
Giancarlo Stanton 122.2 MPH
Manny Machado 199.6
Aaron Judge 119
Shohei Ohtani 119
Franchy Cordero 118.6
We will stop with Franchy, who shows that this stat is still far from a perfect predictor of a hitter's capability, but the top five this year so far includes Stanton and Ohtani again plus Vlad Jr., Soler, and Matt Olson. In general, those that can really square a ball up with a lot a force, tend to be pretty good hitters.
That brings us to Bobby Witt Jr. Right now, he is sitting at 22nd on the max exit velo leaderboard for the year, his highest output at 113.7mph. Do keep in mind that the weather has been cold and so a lot of the velocities are still relatively low, and that is before we get into baseball changes. Salvador Perez is second on the team at 110.8, and Witt has topped 110 three times already this year. The ability to make this kind of contact is not a guarantee that he is the real deal, but it is a solid data point pushing us even farther in that direction than we already were.
Beyond that, you can see how his season has played out to some extent using his exit velocities. At the beginning of the year he struggled, and in his first 15 games where he posted an exit velo, he only topped 100mph in 4 games. There was not a lot of solid contact. In the last 9 games (through Sunday's double header) he has topped 100 in 6 of them including that 113.7 and a 110.2 mph effort. This coincides well with his numbers bouncing back from the rough start. Actually, only Benentendi and Slavy are ahead of him in average exit velocity as well, so he is already catching up to two of the most solid major league hitters in the current line-up.
It is really early in Bobby Witt Jr. career, so statistically most things have not settled in any direction, but one thing we can already say for sure is that his top end exit velocities are among the best in the majors. That is a good sign for things to come.