There are a number of quotes which indicate leadership in some way:
Ralph continued to blow short, penetrating blasts. Piggy moved among the crowd, asking names and frowning to remember them. The children gave him the same simple obedience that they had given to the men with megaphones.
In the above instance, Ralph's leadership is recognised because he is the one blowing the conch. The boys identify him as leader since this is what they had become accustomed to - those with megaphones are to be obeyed. The conch has the same power as the megaphones had. Piggy displays a natural kind of leadership, borne of his pragmatic and logical approach to things as well as his genuine concern for others, as illustrated in the extract. If things had been different, he would have been followed by the boys.
The boy who controlled them was dressed in the same way though his cap badge was golden. When his party was about ten yards from the platform he shouted an order and they halted, gasping, sweating, swaying in the fierce light.
In this extract, Jack's authority stems from the fact that his cap is golden which indicates his authority. He is also ahead of the choir, leading them, an obvious indication of his status as their leader.
Jack started to protest but the clamor changed from the general wish for a chief to an election by acclaim of Ralph himself. None of the boys could have found good reason for this; what intelligence had been shown was traceable to Piggy while the most obvious leader was Jack. But there was a stillness about Ralph as he sat that marked him out: there was his size, and attractive appearance; and most obscurely, yet most powerfully, there was the conch. The being that had blown that, had sat waiting for them on the platform with the delicate thing balanced on his knees, was set apart.
It is clear from the extract that Ralph, in the boys' eyes, seemed a better prospect as leader than Jack, mostly because he had the conch.
In Chapter two Ralph addresses the boys in their first official meeting after the initial gathering. He leads the discussion and states the importance of the conch. He makes the first rule on the island - that the conch is a symbol of authority and should be respected. Also, he is the only one who can intervene - he emphasises his authority as leader.
“That’s what this shell’s called. I’ll give the conch to the next person to speak. He can hold it when he’s speaking.”
“Look–”“And he won’t be interrupted: Except by me.”
Ralph's leadership finds further expression when he states what should be done on the island to be rescued.
“There’s another thing. We can help them to find us. If a ship comes near the island they may not notice us. So we must make smoke on top of the mountain. We must make a fire.”
Once Ralph's authority has been defined and accepted, he continues to lead the boys until Jack eventually challenges him. This results in Jack forming a breakaway group, consisting of the choirboys and others. The only reason Jack's leadership was accepted was because he had offered the boys a more exciting alternative - hunting and because the boys feared him. And so Ralph's dominance was broken.
“Hunting,” he said. He sized them up. Each of them wore the remains of a black cap and ages ago they had stood in two demure rows and their voices had been the song of angels.
“We’ll hunt. I’m going to be chief.”
This was the moment at which Jack became leader of a small breakaway group boys who would challenge Ralph. their actions, under Jack's leadership, would eventually create chaos and result in the shattering of the shell and Piggy's death.