Nothing screams "textbook" like a perfectly formed sentence with zero hesitation marks. Native speakers use fillers to hold the floor while they think. "Like," "I mean," "Well," or "You know." In Spanish: "Este..." or "O sea." In Japanese: "Eto..." or "Ano..."
This is the gold standard. Listen to a native speaker and mimic them exactly as they speak—not after they finish. Aim to match their speed, pauses, and emotional inflection.
Watch the sitcoms, memes, and news that locals consume. If everyone is quoting a specific commercial or a classic movie, knowing that reference is the ultimate "in."