It should go without saying at this point that the vast majority of Japanese pornographic actresses have breast implants. Due to the nature of Asian diets, not only are large breasts rarer than they are in other cultures, when they do have larger breasts they're generally larger women. So, whenever you see a super-skinny Japanese woman with large breasts, they're most certainly fake, even if they seem to bounce like real ones. There are breast implants that do a decent job of mimicking the appearance of natural breasts, but there are plenty of ways to tell that they're fake. Additionally, look for stretch marks on the sides of the buttocks on skinny porn stars. If they have those, guess what? Ass implants.
Even with breasts being fake, if I cannot tell the difference while watching the movie I'm okay with that. It's not like most American models which you can obviously tell how much fake they are at the first frame of the movie.