A friend of ours has a mother who is a messianic Jew. Her mother once suggested to her the way Israelite blood can be detected. She said you can tell an Israelite by their unresting curiosity, by their asking questions and digging to understand a matter. You can tell because they don't submit to authority nor accept answers based on that mechanism. The spirit of a message carries its own authority.

I like her measurement. It may be as good or better than any. Jacob became Israel, and while many of Jacob's descendants were pure blooded, few of them had the "blood of Israel" by this standard. Fewer still had the blood of a covenant.