How can you see that the boys get more and more savage?

Golding really charts out the points on the line of savagery. There are several ways you could pin together the evidence - and see the line from civlisation down to savagery. Have a look at painted faces, at the savage dances after the pig's hunt, or at the breakdown of meetings and the assemblies.

I'm going to show you another way. Here's Roger, throwing stones to miss in Chapter 4:

Roger gathered a handful of stones and began to throw them. Yet there was a space round Henry, perhaps six yards in diameter, into which he dare not throw. Here, invisible yet strong, was the taboo of the old life. Round the squatting
child was the protection of parents and school and policemen and the law. Roger’s arm was conditioned by a civilization that knew nothing of him and was in ruins.

That last line is the really interesting one: Roger is still governed by the rules of the society he has come from. He is still civilised, even though it is apparently subconscious.

Later, Roger is among the boys who takes real pleasure in rolling rocks off a cliff, sometimes causing damage to the jungle, which seems to infuriate Ralph:

A knot of boys, making a great noise that he had not noticed, were heaving and pushing at a rock. As he turned, the base cracked and the whole mass toppled into the sea so that a thunderous plume of spray leapt half-way up the cliff.

Then, even later, it is Roger who stabs the sow "right up her ass":

Roger ran round the heap, prodding with his spear whenever pigflesh appeared. Jack was on top of the sow, stabbing downward with his knife. Roger found a lodgment for his point and began to push till he was leaning with his whole weight. The spear moved forward inch by inch and the terrified squealing became a highpitched scream.

Golding describes it savagely - it's clear that, even though he's not yet hurting humans, Roger is becoming less and less civilised. This is a cruel, undignified slaughter of an animal. Then, by the time of the scene at Castle Rock, Roger has become entirely undignified. As Samneric approach, he first throws stones to miss, as he did back in Chapter 4:

Roger took up a small stone and flung it between the twins, aiming to miss.

As the pressure builds, Ralph notices that someone is throwing several stones:

Someone was throwing stones: Roger was dropping them, his one hand still on the lever.

Finally, Roger throws caution to the wind and rolls a massive rock down from his platform:

High overhead, Roger, with a sense of delirious abandonment, leaned all his weight on the lever... The rock struck Piggy a glancing blow from chin to knee... Piggy fell forty feet and landed on his back across the square red rock in the sea. His head opened and stuff came out and turned red. Piggy’s arms and legs twitched a bit, like a pig’s after it has been killed.

I think you see from the build up of rocks and of pig-killing how Roger's savagery builds. This is just one of the boys: but I think he encapsulates the descent into violence, fear and savagery extremely well.