how do we see that jack is becoming the leader of the tribe?

Jack openly contradicts Ralph during the meetings. While Ralph is painfully trying to convince the boys that there isn't a beast, Jack wants to hunt and kill it. Jack makes the boys feel safe under his bravado while Ralph increasingly sounds weak. Jack challenges Ralph's authority at every turn and Ralph is unable to fend off his attacks. When they go and search for the beast it is Jack at the head of the line of boys.