Actually, there are. It is a multidisciplinary effort. There are no definitive tests yet to objectively determine that, but the science and the psychology are well known. Your skepticism has nothing to do with the scientific reality. the area is complicated by the fact that not everyone who is transgender is binary. There are many non-binary and gender nonconforming people falling across a wide spectrum. These are even harder to test if they were in objective test available. Their results would be mixed.