Evaluating pitching starts with velocity, which needs to be at around 85 miles per hour to get a scout’s interest. After that, things get a little murky because of sabermetrics.
Can they throw multiple pitches? Though pitch variety is worth noting, it's not as significant as how often they get people out.
The subcategory sabermetrics eliminated was evaluating delivery — an almost entirely subjective evaluation.
Scout's take: I don’t think we evaluate pure arm strength like we used to because of how rotations work now. The starter goes five innings, and four guys come in to finish it off who all throw around 95 (miles per hour) and all have one pitch that’s just filthy. Most guys just don’t have the pitch range to make it through the order three times, but if you can throw 95 and your control isn’t great, there is some room to work there. You can’t ignore that type of velocity.
Most amateur pitchers aren’t going to be able to accurately throw every pitch. But I do think they should have a certain type of pitch they’ve mastered. Another big thing I look for is spin rate. If it’s not spinning but it’s still playing, you hold out hope, and sometimes, those guys can really make the ball jump on you at the plate. But if it’s spinning and moving already, that’s great.
I certainly think delivery is something that can be developed, and unless they’re doing something super funky, where you think it might eventually cause injuries, you won’t walk away from a guy with good arm action because he’s got a delivery that’s not traditional. If you’re evaluating pitchers, what you really should be doing is talking to hitters. Find the guys they pitched against, and they’ll tell you who has what it takes and who doesn’t.