GENETIC MARKER
It's very very simple, and now Israel DNA tests certain people who want to immigrate. If it didn't exist and was not accepted by them as legitimate, they wouldn't test for it and use it as the defining issue in those cases.
There are jews of every race: chinese, indian, african, etc. They have the GENETIC MARKER.
It's not conversion, which is why that is not, was not, and will never ever be accepted by jews: its that GENETIC MARKER.