What are the indicators of insufficient decoupling
The symptoms are that most of the time everything will be fine, except sometimes it might not be.  This can be data-dependent and very hard to reproduce.

Think about what's happening.  Some chip suddenly increased its current demand.  That caused its immediate power voltage to dip to some level where correct operation is no longer guaranteed.  Even if not, the rapid change of the power voltage can cause trouble.

It is very hard to predict what exactly that trouble might be and at what threshold of voltage or derivative of voltage it occurs at.  A data line may be temporarily interpreted in the wrong state.  A flip-flop may get flipped.  You don't know.  Whatever happens is also a function of temperature, even uneven heating of the die.  Try reproducing that exactly from one test to the next.

So the bottom line is things can get flaky.  Maybe.  Sometimes.