They could use their own eyes - and go to a supermarket.
Cabbage, Brussel Sprouts, Kale, Broccoli, Cauliflower - They were all one plant before humans came along and started selecting and breeding them into different species.
They could use the same DNA test, that anyone would use in a paternity case - to test how closely related a father might be to a child but instead use it on themselves and a chimp or gorilla. They’ll be able to realize that the chimp or gorilla is a cousin, just many times removed.
You could go to a natural history museum and show them all the transitional fossils that are half way between species we know now, and ones we’ve found in the past.
You could point to the creationists own children and remind them how similar the child looks - how it exhibits traits from both parents, but also has some uniqueness.
You could point out - that if an entirely alien species came to planet earth - it would look at all life and in a matter of seconds and armed only with the intelligence of a 5 year old - be able to see which species are related. How could anyone, fail to notice that we share veins, hair, skin, brains, eyes, ears, lungs, hearts, nerves, arms, legs, heads, muscles, blood, kidneys, bowels, mouths, stomachs and so on and so on - with all the other mammals.
How could anyone not look at all of these things, and not see in a second that we’re related to all of the animals - and that we have evolved different traits based on survival.
Cults. Cults and indoctrination into a cult, that teaches you from childhood that its all not true, is the only reason why evolution is not self evident given what we now know about DNA, farming, cultivation, cell division and so on.
Only a cult - could possibly maliciously and deliberately try to subjugate and ignore something which is so self evident.
