The best way to figure out if you are on a route (usually) is if the protagonist spends a lot more time with the heroine. In the case of Hoshizora, you seem to enter a route after a short monologue the heroine might give.
Also Asuho asks about the mystery girl a ton.
I'd skip the route by the way. The best routes for me (in order) is the mystery girl, Komomomomo, and Isuzu.
However, don't skip, just skim. You will miss out on why certain characters exist and such, and with Chinami, you will miss out on Kogasaka's backstory.