David P. answered 03/26/19
Experienced photographer, cinematographer, actor, and VO talent
Generally, it's going to be a combination of a good SFX team, color grading, and on set effects. If the color grade and lighting match between the set and what is layered on the green screen, it can be difficult to tell if it's part of the set or not. Layer that with an on set environment that matches what the green screen shows, and the SFX will be difficult to pick out.
In newsrooms they often don't have the time or need to match the color grade and lighting effects. No one expects the green screen footage on the news to look like they are actually there, and the time and cost of changing the lighting setup would be prohibitive to actually telling the news.
For most YouTube producers, the budget isn't there to be able to either: 1) film the needed green screen footage themselves, 2) have the lighting setup on set to match what was filmed, 3) get stock footage to use that was either shot in RAW (which isn't common to begin with) or matches what they can replicate with their studio lighting. Or it could be a combination of those factors.
Another factor is environmental interaction. If the wind is blowing in the green screen footage, but the actor isn't affected by any wind, it will look fake even if the lighting/color grade match. Having scenery and environmental props for the actor to interact with can also help set the scene. If the screen is showing footage in a forest but there are no trees or foliage around the actor, it's going to look fake. If the actor is in a car, and the footage used for the green screen is showing that the car is driving down the road but the car is perfectly still (no bouncing from potholes, etc...) the scene isn't going to look realistic.
If everything is done properly however, it can be difficult to tell that a scene wasn't shot on location, or that SFX were added at all.