No source named
Who said it? If nobody is named, that is your first clue.
A source is the person, group, or place a fact came from. A scientist, a news reporter, a museum, a teacher, a book.
Real reporters and real experts show their work. They say "according to NASA" or "Dr. Lee said" so you can go check for yourself.
Writing with no source is not always wrong. But it is asking you to trust it without proof. A detective never does that.
Spot it
“Experts agree that kids who play outside for one hour a day get better grades.”
Which experts? Where did they say it? There is no way to check.
Detective move
Ask "Who says?" Then search the main claim with a grown-up and see if a trusted site says the same thing.