Answer:
Evolutionists determine that two organisms have a common ancestor is by looking at fossil evidence in different rock layers using the law of Superposition (Oldest layers are on the bottom, newest are on the top) and compare the skulls or other bones to each other in order of oldest to newest (or newest to oldest). Another way to determine this is to examine the amount of DNA a certain species shares with another species. An example of this would be that Humans share roughly 90% of our DNA with chimpanzees or the other Great Apes.
Step-by-step explanation:
DNA
They can look at the DNA it's the most common one.
There are 4 pieces of evolution and they are
Fossils , Geography , Embryos / DNA , Anatomy
Fossils: Physical remains of species , Determine age, location, environment
Deeper layers = older
Geography: Proves species share common ancestors, depending on where
they live
DNA: BEST evidence because it’s the MOST ACCURATE
Similarities in the early stages of development
Similarities in DNA
More similarities = closely related
More differences = not related
Anatomy: Compare body parts of different species to see how they evolved
3 different structures:
Homologous (same structure, different function)
Analogous (similar structure, different organisms)
Vestigial (body parts that no longer serve a purpose)
All of that are in evolution
Hope it helped! ( Gave u my biology notes :D)