I think it's pretty obvious when an ass is natural, an ass that big specially. Girls built like that are usually fat at some degree, with very wide hips and a big belly(the ones that don't have belly usually undergo liposuction, use corsets or have very good genetics)
Plus you can also notice it on how the flesh jiggles. If it jiggles a lot it's most likely natural, although idk how good BBLs can be right now.