When you head out to scout a low minors team, the first thing you need to do is invest in a program (or print off the rosters, with player numbers included, before you get there). They don't wear names on the backs of the jerseys down at this level, so it's going to be hard to pick the guys you want to look at out of a lineup. This is more important that picking your seats, honestly, and having a good idea whether there's some potential top 100
Next, get there early and watch players play catch if possible. You'll get a good idea of a pitcher's athleticism if you're able to watch the pitchers playing catch in the outfield. Same goes for batting practice. Many times they'll be shagging balls in the outfield or even hitting fungoes to infielders. Doesn't hurt to get a look at how they move around off the mound as athletic pitchers, as RB often notes, have an advantage when it comes to repeating delivery and mechanically putting it all together.
Now lets look at some attributes:
You'll generally have a good idea of a players height and weight from the roster or the program, but seeing them in person can be helpful. If the pitcher is young and has a small frame, does it look like he's fully mature? Or is he going to be able to fill out his string bean frame a bit more as he gets older? If a player is 6'3" but only 165 pounds, can he add strength and size or is he going to look like Alexei Ogando for his whole career? Projection can be hard and is an inexact science but you can generally get some idea by taking a good look.
Where does the pitcher release the ball from? Does it come straight over the top? Or 3/4 of the way between the top and sidearm? Pitchers who throw over the top will generally have a bit less horizontal movement on their fastballs but also generally have the ability to throw better curveballs. 3/4 arm slots usually get a lot of horizontal movement, but usually end up throwing a slider, cutter, or changeup as their secondary offerings as curveballs from that slot generally have more horizontal plane and less depth, making them easier to hit. Both can be successful and neither is mechanically unsound, but the biggest issue is whether the pitcher can repeatedly throw his pitches from the same armslot, be it fastball or secondary offering. If the arm slot and release point are changing, not only can that tip off the pitch that is coming but it can also lead to injury down the road.
This is the big one -- if you're throwing 95+ on a regular basis you're going to get a lot of attention from scouts, but it's not the only thing and probably gets a little more attention than it deserves at times. Still, good velocity affords any pitcher room for error, both within the strike zone and with secondary offerings. Can you be successful without it? Certainly, but he road gets a tad bit tougher at each level. Can you be successful solely because you throw 100 mph? No, likely not. You need some combination of the other traits I'm about to talk about as most pitchers will eventually get the point where anyone can hit a 100 mph fastball if they know its coming.
First, how many different types of fastballs does a guy throw? Is he a four seam guy or does he throw a four seam and a two seamer? Does he also throw a cutter? It can be hard to tell when watching the game, so having this information before hand is helpful.
Four seamers generally don't have a ton of horizontal movement too them, but you don't want them to "ride" as they approach the plate -- you'd like to see good plane and hitting the bottom of the strike zone with the four seamer if possible.
Two seam fastballs will tend to show some tail to the pitchers arm side, cutters will generally tail towards the glove side as a pitcher applies some finger pressure to make the ball move that way.
The bottom line is that movement is just as important as velocity in most cases, especially if a pitcher can harness it to his advantage. Things to look for are how often he's able to find the corners using the movement on his fastball in given outing.
Basically: Can he throw his pitches for strikes? If you throw 99 mph but can't ever find the strike zone, you aren't going to be much use to anyone really. This is more true of breaking stuff than fastballs, honestly, but I like to be able to see a pitcher throw his secondary stuff for strikes when watching him pitch. That can be the difference between a guy making it or not a lot of the time.
An extension of control, command is the ability to throw pitches where you intend them to go. There's a big difference between splitting the plate with a fastball and riding the black on the edges, but both are strikes. Same goes for being able to bury a curveball or slider down and in or throw a back door breaking ball that catches the plate in a 2-0 count. When I think of lack of command, I think of Seth Blair or Kip Wells -- they've got velocity, great secondary stuff, but they can't throw the ball where they want, which means they get hit hard an awful lot.
A word you see associated with curveballs a lot is "depth", which is the extent to which the pitch moves downward and falls off plane. Really good curves are thrown from a higher arm slot and will break in a 12:00 - 6:00 or 11:00 - 5:00 manner if you were watching the pitch move on the face of a clock. Things to look for: How big is the break? Does it move vertically more than it does horizontally? Or does it tend to flatten out and move less on the vertical plane? Curveballs that flatten out are "hanging" and can get tattooed by hitters for extra bases a considerable chunk of the time. I generally like to see a pitcher "back foot" a curveball a couple of times in a game, meaning he's able to drive the pitch to a glove side hitter's back foot. You can't accomplish this without good depth on the breaking ball.
Sharp, late break is what you are looking for here. Good sliders, like Greg Holland's, move almost like a split finger fastball: They're hard and they break sharply down and to the glove side late in the ball flight. Bad sliders are ones that "back up", or basically spin with very little movement, usually due to slipping out of the pitchers hand. They will "back up" and stay hanging out on the pitchers arm side, ripe for driving over the wall by most good hitters.
Good changeups will have "fade", which is movement down and away to the pitcher's arm side. If you watch Michael Wacha's changeup, one of the best in the game, you will notice that it will move from the center or arm side corner and start fading away as they get to the plate. That, coupled with the change in speed, is what makes the pitch so effective.
The key for the changeup is the differentiation in speed from the fastball and keeping the arm slot and release point exactly the same so the hitter can't be tipped off that the pitch is coming.
Mostly, this is pitch sequencing or "feel for the game". Pitchers who have this ability seem to have it innately, but it can also be the product of strategic planning on how to get a hitter out. Basically, the pitcher is able to throw pitches that the batter isn't expecting and to spots where he's not expecting them. Think Adam Wainwright throwing back door breaking balls to a left handed hitter on the first pitch they see, and then starting them off with a cutter in on their hands in the next PA. That's pitchability.
What's important here is not so much the quality of the secondary offerings, but the ability to throw them different speeds and command the pitches so they go where you want. By keeping the hitter off balance, you don't have to have amazing stuff, but you can get hitters out based on being in their head and setting them up so that the pitcher only has to throw his pitches to get outs rather than coming into the hitters portion of the strike zone.
Tim Cooney is a model of pitchability, in my humble opinion -- he doesn't have any plus stuff, but has four pitches and keenly knows how to use them to keep the guy with the bat guessing.