Their negroid appearance is the first give away, but I believe they actually got genetically tested and it was revealed they're like 50% Canaanite generically. Juda's sons did marry Canaanite women, but after the Exodus, race-mixing, especially with Canaanites, was forbidden by the Law. The Canaanite admixture should have gotten bred out of the Judahites by mixing with the other Hebrews, especially since the Second Temple Jews were a mix of Judahite, Benjaminite, and Levite (and of course some Edomite converts, but Edomites were a Hebrew race). Of course, the fake jews of today cope by claiming that the Israelites were *always* Canaanites and originated in Judea (which again, directly contradicts not only Scripture, but also a good deal of ancient Historians).