What is the sign of the covenant?

The outward sign was, and still is, the brit milah, which is the covenant of circumcision.
Another outward sign of God's covenant with the Israelites is the Torah scrolls which the Israelites (Jews) possess. The scroll is called the Book of the Covenant (Exodus 24:4), the Ten Commandments are called the Tablets of the Covenant (Deuteronomy 9:9), and Judaism as a whole is called the Covenant (2 Kings 23:3). See also the Related Links.

Link: More about Jewish circumcision

Link: Facts about the Torah-scroll

Link: What is the covenant of the Israelites?