How do I detect a real trailer?

Given the ease of use of modern editing software and the availability of high-quality source files, "fake" trailers can look as good or better than "real" trailers. One sign of a "fake" trailer is when the action and cuts don't match the beats of the trailer's soundtrack, though a good editor will avoid that.

One solution is to go to the film studio's own channel to see if the trailer is posted there. For example, there are a number of channels that repost Marvel Cinematic Universe trailers, remix them, or edit them together in a series to make it look like a longer clip, but the Marvel Entertainment channel at MARVEL should always have the real trailer for any upcoming MCU film.

All of the major studios have their own channels; I subscribe to most of them specifically to avoid the issue you're talking about.

Another approach is to manage what channels YouTube uses to recommend videos to you. If you notice you're getting recommendations from sub-par channels instead of the actual sources, you can click the three dots just below the thumbnail to the right and select the "Not Interested" option.

When you get the response "VIDEO REMOVED," choose the "Tell us why" option and choose the "I'm not interested" in this channel option.

After you do this enough times you won't be getting the most common undesired recommendations.

(Note, I'm a fan of Screen Junkies, so I'm just showing this for illustrative purposes.)

Another approach (recommended by Ankit Sharma in the comments) is to check to see whether the channel posting the video is verified by YouTube as "belong[ing] to an established creator or is the official channel of a brand, business, or organization," which is indicated by a verification checkmark:

Note that this channel lacks the checkmark:

This isn't foolproof, since many channels that repost trailers are verified, but you may see material posted by someone with a lookalike name such as "Marvel Movies" that doesn't have the check.

Since people today do ANYTHING to get views, like using attractive titles and highly custom cover photos for their video, I'd suggest that please refer the video-poster's name which always flashes under the videos title.

A very few people write '(Fan-made)' in their video titles but please note that many don't, as it's not a compulsion imposed by Youtube and they all work around them views.

As for my answer,I would never open any MCU-related video unless it is posted by Marvel or any connected(trusted) partner.