This article is from the RSC/UKSC Cricket FAQ, by Ganesh with numerous contributions by others.
Thanks to: Ron Knight and Ian T. Brickbats: Me!
Umpires are taught to ask the following 4 questions in order:
1) P - Where did the ball Pitch? (if outside legstump means not out)
2) I - What did it Impact? (must not be bat or hand holding bat)
3) P - Where's the Point of Impact? (See explaination below)
4) S - Would it have hit the Stumps?
The 4 questions to be asked are remembered by
noting the following mnemonic - PIPS.