Using various paleoclimate proxies such as ice cores and sediment samples, paleoclimatologists have been able to pin point events such as the Permian - Triassic extinction event 251.5 Mya, the K-Pg boundary 66 Mya when an asteroid / supervolcano killed the non-avian dinosaurs and ended the Cretacious, Paleocene–Eocene thermal maximum 55 Mya, and many more. They get more and more accurate the closer to modern time they get.

A mass extinction caused by a nuclear event would not go unnoticed- especially as recently as the pleistocene. There would be evidence of decaying or decayed radioactive isotopes.

If you are interested in such things as past human-oid civilizations, I suggest you put your mental energy into learning about our closest relatives such as Neanderthals, Denisovians, and Homoflorensis. Or more distant relatives like the Ardipithecus family from ~ 7mya. Truly fascinating stuff.

Otherwise, if you continue on this conspiracy theory path of "past nuclear civilizations" based only on the fallacy that it's impossible to prove something DIDN'T happen; you will end up another ancient aliens nut case or Lock Ness Monster truther and deprive yourself of a whole world of fascinating REAL knowledge about the world and it's past. Even fueling harmful controversy for real researchers trying to conduct real research about past civilizations. Please don't be one of those people...

Here's a PBS Eons video to get you started in the right direction if you enjoy learning about ancient humans

Edit: also, head over to r/AskAnthropology if you have more questions