Correct, it's hard especially proving it true (or false!). But you can analyses many aspects of it in isolation, e.g. short term reactions of very young childr

Correct, it's hard especially proving it true (or false!). But you can analyses many aspects of it in isolation, e.g. short term reactions of very young children to subtle subconscious mimic and gestic from people around them. Similar you can somewhat study how trauma is passed onto future generations in some cases. You can also study how can e.g. realize a subconscious fear of the parent related to something then tend to develop that fare, too. Etc. Etc. In the end you basically end up with many puzzel prizes which happen to fit very well together, but due to the limitations of science you can't prove that they fit. But there is no indication that they don't fit and logical reasoning says it's very plausible so acting on the assumption of it being right is quite reasonable.