Napoli have not tasted defeat in the league yet, although they have not yet faced a threat to them. However, I do not think they need to test their strength after winning the title last season. Napoli are still a very strong team, but we will see how strong they are this season in the next matches. In the next match they will face a strong team AC Milan, but it will be at AC Milan's home ground. In this match we will be able to test Napoli's strength
Yes, the match against AC Milan should be a test of Napoli's true strength. And we'll see how far this team can go against a team of equal strength. However, if Napoli can win against AC Milan, it will be difficult to find a team that can beat Napoli this season. Perhaps Juventus, Inter Milan, or AS Roma could have a chance to match Napoli. But if Napoli also beats those teams, it looks like this season could be Napoli's again.
However, there's a difference this season: Napoli's focus this time is also on the UCL, so that should cause Napoli to lose focus on the domestic league as well.