Legally, you are responsible for ensuring YOUR work follows copyright. If your work is made of other works then you need to ensure you have a license for them all. Otherwise you can be sued and saying someone uploaded it to Flickr with a CC-BY won't save you. Presumably you'd track down the original author and have them sign a piece of paper where they claim it is their work. Then you can at least sue them for damages if it's not.
edit: As for your example, copyright generally doesn't care about you consuming a work but about you sharing a work. As such, you don't have to check anything but Disney does have to check everything.
That’s what I meant. The youtube poster is responsible for the copyright. I don’t think it’s possible for me to check beyond the legal copyright.
Disney, and random youtuber, does have to check everything, not me if I’m legally using it. However, if the copyright if wrong, I’m not sure what the liability is.
edit: As for your example, copyright generally doesn't care about you consuming a work but about you sharing a work. As such, you don't have to check anything but Disney does have to check everything.