As is the case with position players, the first thing scouts look for in a pitching prospect is a strong arm. The reason why arm strength matters more is because it's something you are born with. Strength can be improved on slightly, but no matter how much time he worked out either on the field or in the gym, Jamie Moyer could never throw 95 because he didn't possess the God given ability to do so.
The same principle holds true on other varieties of pitches as well; things like length of the fingers and forearm, strength in the hands, and flexibility in the wrist all determine how a pitch is thrown and how it will react. A perfect example is Mariano Rivera's cutter, he was a struggling starter who lacked a dominant pitch who accidentally discovered the holy grail. All it took was combine his natural release angle and throwing motion with a different grip and he went from a guy who was almost traded to a Hall of Famer.
When scouting a pitcher for the first time, a scout will use the radar gun to establish a baseline for future viewings, either his own or a crosschecker or supervisor from his own organization. What's really important isn't the raw velocity, but his ability to "carry" the velocity deep into the game. The same applies to his secondaries; if his curve is 76-79 in the first inning is it the same in the fifth, and if not is there a noticeable difference in shape or location.
Recently, former Baseball America writer Conor Glassey and Jason Parks of Baseball Prospectus have taken heat for criticizing fans who go to games then use blog forums and social media pretending to be scouts. Parks especially took more than his fair share of abuse, which is unfortunate because he happened to be right with most of what he said.
The truth is, most of these "fan scouts" don't know why real scouts use radar guns and lack the experience and skill needed to properly identify pitch type. The truth is, at the professional level, scouts are assigned to "areas" or certain teams and for the most part have had multiple looks at the guy on the mound. This means their "baseline" has already been established; what they're doing now is comparing...is his fastball the same in June as it was in spring training, if he's working on a slider has it shown improvement since his last viewing.
Another common error, especially with the fastball, is believing the sole component to the grade to be velocity. It is important obviously, but more important is movement and command. Major league hitters have no trouble with velocity, if the pitch is straight they'll eventually time it, but if it has even the slightest movement the odds of quality contact will greatly reduce.
There are three types of fastball; the four seam, the two seam, and the cutter. The primary difference between the two former is the grip; the four seam is across the seams (2 fingers across 2 seams equals 4), the two seamer is with the seams. The four seamer has the higher velocity and at a higher speed gives the batter the illusion it is rising. The two seamer is thrown with slightly less velocity because of the drag on the seams and will fade to the pitcher's arm side as it approaches the batter. The cutter is thrown similar to the two seamer with the difference being the index finger isn't on the seam but in the space between them. The slight variation in grip and in pressure on the middle finger causes a slight cutting movement similar to a slider but not as pronounced.
When grading a curveball, scouts look for tight spin which leads to a tight break and both lateral and downward movement. A slider is a higher velocity pitch; while the break is lateral like the curve, the movement is more horizontal than vertical.
The fourth most common pitch type is the change of pace, often misdiagnosed as a change-up. The term change of pace is used to describe a pitch that a hurler will throw maybe ten times a game just to give the hitter something to think about during his at-bat. Roy Halladay's change of pace was his cutter, he'd throw 80% fastballs during a game, 15% sliders and 5% cutters.
A true change-up, like the one thrown by Trevor Hoffman, is the single hardest pitch to learn because it requires an almost freakish ability to keep the fingers out of the delivery. A true change is held in the palm of the hand under the ring finger (where there is less strength), yet is thrown with the same arm speed and angle of release as a fastball. I would venture a guess that out of the estimated 1800 professional pitchers there are less than 100 legitimate change-ups and fewer than maybe 20 that would be graded as plus.
The pitchers delivery is also a factor in his grade, you want to look for a smooth, repeatable motion with little movement throughout. You don't want to see things like throwing across the body (with apologies to Jared Weaver), a stiff front leg, overstriding, arm lag or the plant foot landing on the heel instead of the balls of the foot.
Release point is another factor, and outside of an inconsistent delivery is the leading cause of command issues. I'm sure you've heard term "throwing out the window", that means to the batter each pitch is released exactly the same. The advantage swings to the hitter if he's able to determine even the slightest difference if the curve is thrown out of a different window than the fastball. Some BOR pitchers will compensate for their lack of quality stuff by dropping down occasionally or altering the speed of their delivery (also a change of pace) to keep the hitter off balance.
Major League average for a fastball is about 90 mph, so you'd grade that on velocity alone as a 5. You would then look at your tracking sheet and look for his strike percentage (control) his ability to throw the pitch with confidence in any count or situation (command) and his ability to carry the velocity throughout the game in order to come up with an overall grade.
You would then do the same thing with his other pitches. Stephen Strasburg's curveball is in the 88-90 range, harder than most guys slider and close to the average ML fastball. Moyer's curve was probably 10-12 mph less, but if he commanded his better and has a higher strike percentage his would be graded higher despite the naked eye telling you otherwise.
I was not a pitcher when I played, and while I've been fortunate enough to be able to talk pitching with Hall of Famers and other experts in the field, it's not my strong suit. That said, I know enough to realize that what is being presented on various blogs and Twitter accounts is misleading, unintentional as it may be. It's all well and fine if you want to sit behind a scout at a ballgame and copy down what's on his gun, but not knowing what the scout is specifically doing that day or even the purpose of the numbers themselves leads to misinformation.
So, the next time you go to a game, watch the game. Do what scouts do in the field. Watch a pitcher for an inning or two from the first base side, then switch to the opposite side. Go down the outfield lines as far as you can, I know most minor league parks don't have outfield seating so getting a true view from the back is nearly impossible, but close is better than nothing. Go sit behind the plate, but focus on the release point and on the ball itself as it travels towards the plate. You know the best way to see how good a pitcher's stuff is? Watch the hitter, he'll tell you.
A good friend has been a scout for a long time and was a successful minor league pitcher before the dreaded rotator cuff bug bit him. He said when he scouts pitchers, even established major leaguers, his primary point of focus is the plant foot. Ideally, the foot should land at the same angle as the slope of the mound, with the balls of the foot touching first, then the heel, with the delivery and follow through passing directly over the now secured foot.
He told me any changes means the pitcher is compensating for something, either fatigue or an injury or an attempt to regain lost command. After ten pitches or so to start the game a telltale mark appears on the mound, he focuses on that and notes any differences as the occur, and subsequent to that the result of the pitch.
These are the things scouts look for, and are the things that matter.