one way to test it is to take a group of fossils and check if they fit with their phylogeny. now, take a look at this figure:
(image from “Molecules consolidate the placental mammal tree”)
as you can see- about 50% of the cases in the phylogeny that base on the morphological data arent fit with the their position that base on the molecular data. now, take a look at this tetrapod evolution (image from wiki):
remember that this series is base on morphology since we dont have genetic data from these fossils. it means that about 50% of the fossils in this figure arent realy fit with their supposedly real phylogeny that base on the molecular data, and thus are “out of place” actually (so a primitive fossil can appeare after more modern fossil). and this is even when we ignore the “out of place tracks” in the bottom left. remember that this is a single group out of many, so we actually have hundreds of out of place fossils.