Penalties are most often thrown for playing on. A player who continues to play with an obvious hit he could have checked without the assistance of a judge will receive at least a Major penalty. A player who continues to play with an obvious hit he could not have verified without a judge will receive at least a minor penalty, however this could be assessed as a Major as well, and generally is because the player should have known he took a hit and asked a judge to check him.
A player who continues to play with an unobvious hit will receive at least a Minor penalty.
Of course, to understand this, you need to know what is and is not considered obvious.
NOT Obvious: Pods, very top or front of hopper, pretty much it.
Everything else is obvious.
If a player is shot in the hopper and it sprays onto their mask and they continue to play, it will be assessed as at least a Major penalty.
Other things can give penalties as well. Interference with a judge during game play. Too many layers. BPS/FPS in excess of 12.5/300, respectively. Unsportsmanlike conduct (This is a big category and includes everything from giving another player the middle finger, to ridiculous amounts of overshooting, to cursing loud enough everyone can hear it, etc. Sportsmanship is a blanket statement and I feel players understand well what is and is not sportsmanlike behavior. As such, this is generally assessed as a major penalty, but it is not always so.)
Really, for the best answer to your question, read the PSP Rule Book.
You can find it right here.
Specifically, read Section 4.2 at least, if not all of Section 4. It begins on page 10 and gives a very clear definition of what is/is not a penalty.