A basic technique used when
troubleshooting electronic circuits.
Simple: You start with a known input and output for when your circuit is working. (You might get these off a block diagram of some sort). Check your signal at input and output. If one is bad (usually the output...) then check halfway between them. Always check between the good input and the bad output... so if your circuit goes 1, 2, 3, 4, 5, check one and 5, if five is bad, check 3, if 3 is good check 4, if it's bad check 2.
Couldn't be simpler. Beats the hell out of easter egging.