I think the poster is responsible for correctly releasing content under the right copyright.

I think it’s reasonable to assume that because something is labeled CC-BY that it’s legal to use for situations allowed by CC-BY.

Is there some copyright certification service? Is there a way to test copyrights? If I don’t go my the author’s copyright, how could I verify it?

If I watch a movie, should I not do so until I verify the personality rights of every person who appears in the film? How can I trust Disney?

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.

While I understand where you’re coming from, in terms of the current legal situation, you are liable for mistakenly believing some license information when that turns out to be wrong.

The way that publishers, for example, avoid such pitfalls is by exclusively working with professional agencies that control the provenance of the work they license.

Watching anything isn’t copying nor showing and therefore safe. Practically, if you want to use any CC-licensed photos, I would advise at least a reverse image search.

One way to protect yourself is buying licenses to images from a service that provides legal indemnification. Part of the reason why stock image websites can earn money is that they provide this, so (at least part of) the risk is with them.