After which event could you say with most certainty that evolution has occurred?

Answer 1

I'm assuming that you mean to ask when it becomes clear that a particular population has evolved, right?

The answer to that depends on the level of detail at which you observe the population in question. If you're observing on the genetic level, then a single generation worth of observations would suffice to show that evolution has occurred. If you're limited to the level of morphology and behaviour, then more generations might be required for the changes to become visible. Each of these findings would be equally certain.

However, if you are looking to point out an example of evolution to a layman, then an instance of speciation would probably be the clearest, specifically an instance of speciation that has led to a near complete decline in interbreeding frequencies and is also associated with a divergence in behavioural and morphological features.