The following picture shows a "tilt meter." You place this on the playfield after you've leveled the game horizontally (put a level from left to right on the playfield and ensure it is level), and look for the pointer to be pointing anywhere from 5 to 7 degrees. The steeper the incline (towards 7 degrees), the faster the ball will roll down the playfield, the shallower the incline (closer to 5 degrees), the slower the ball will roll down. Some arcade and pinball route folks set the game up really steep and cause the ball to roll down really fast. The downside to this is that it wears out the playfield when it's too steep (remember, they don't want you to keep playing too long). Of course if it's too shallow a pitch, the ball will seem to just bounce around hitting pop bumpers taking forever to roll down the playfield (and easier to rack up high scores I might add). Personally, I shoot for around 6 and a half degrees.
(click any picture to enlarge)
This next picture is of a different kind of pitch tool. It has a vernier caliper that is more accurate than the tilt meter shown above. It isn't all that hard to read and the degree readings are spaced apart a little more. The downside is that they can only read about 20 degrees whereas the tilt meter can go 180 degrees and can be used for other reasons ( not just for pinball games, for example, a satellite dish).
There is of course one other method if you don't have one of these tools, and that is to simply screw out the leg levelers to about 2/3 of the way on the back legs, and screw the front leg levelers all the way in. After screwing the levelers in or out, just level the game horizontally. Don't use the bottom of the game box since you really want to go by the playfield being level rather than the game box (they don't always match up - that is, the game box can be level, but the playfield isn't). I actually start with leveling the game box horizontally and then do a final check with the level on the playfield to see if it too is level.
Hope this helps, let me know if you have any questions (it's really rather easy to do).