How can you tell if the nonfiction you reading contains opinion?

Lets use Taylor Swift as an example: If the author States something like "She is awesome" or "She is the best singer". The author is biasing her liking upon the character. But, of course, not ALL people think Taylor swift is awesome or the best singer. If you wanted to see a non-opinionated statement, it would say something like "There are many people that Taylor Swift is the best singer but many others do not". That statement isn't biased. I hope this helps.

As the boys sleep, military airplanes battle fiercely above the island. none of the boys sees the explosions and flashes in the clouds because the twins sam and eric, who were supposed to watch the signal fire, have fallen asleep. during the battle, a parachutist drifts down from the sky onto the island, dead.