Archaeologist can find that fires were started and re-started, Archaeologist can determine if these were done by small groups or large groups, but without written records we can NOT see by archaeologist reports WHO set the fire. Just think about it, how would a Christian set a Fire Differently than a pagan from his same class? Used holy oil that leaves the sign of the Cross? Fire is Fire.
Tacitus reported that people did see people stop people from putting out the fire, Tacitus reports people saw other people re-starting fires that had been put out. Tacitus then say Nero blamed these acts on the Christians in Rome. That is the ONLY Contemptuous written record we have on the fire (and even it was written AFTER Nero was no longer Emperor). To jump to the conclusions that Nero had been right in blamming the early Christians based on evidence that the fires were re-started over and over again is a violation of any rule of logic I know. Tacitus clearly says that Nero picked on the Christians ONLY after people started to blame him. Christians were an easy target, they were viewed as Jews and Rome was in the middle of putting down the Jewish Revolt of 70 AD at the time of the fire. The Early Christians had no real power in Rome of that time period so an easy target. In fact the Jews living in Rome probably saw the Christians as Heretics and helped out Nero in looking for Christians (More to show their loyalty to the Empire and Rome given that the Jews in Judea were in full Revolt).
Sorry, given the situation in Rome at the time of the Great Fire, only one person had the ability to get a large group of people to keep a fire going, and that was Nero himself. His Palace minions would have done it just to make him happy. Since the time of Augustus Caesar, fire suppression had been an Imperial responsibility
People forget how Marcus Licinius Crassus Dives (Commonly called "Crassus"), the Third man in the First Triumvirate of Pompey the Great and Julius Caesar, had made his money. Crassus set up the first Fire Department in Rome. His fire department would appear at a fire and buy the building from the existing home owner who preferred to get something rather then see the whole building burn to the ground. Crassus's fire department would then put out the fire and Crassus would have a building at the fraction of the cost if he had built it himself. For more on Crassus:
http://en.wikipedia.org/wiki/Marcus_Licinius_CrassusAnyway, NO Ruler after Julius Caesar were going to permit someone, anyone, to get as rich a Crassus had become do to his control of the fire department. Thus from Julius Casar onward Fire suppression in Rome was an Emperor Concern, and at the time of the Fire the Emperor was NERO. Thus the only person who COULD have permitted the fire to spread was NERO not the Christian even if they were large enough in numbers to do spread the fire (A fact most historians doubt, putting the Number of Christians in Rome at the time of the Fire in the Hundreds maybe a couple of Thousands at the most in a city with a population over 1 million).