In most cases you can just get rid of everything starting with the first question mark you see. There are some exceptions, like older YouTube links.

Example:

Old link: https://youtube.com/watch?v=dQw4w9WgXcQ

New link: https://youtu.be/dQw4w9WgXcQ

You can see that while the new one just uses the video's ID as its own page the old one uses it as an identifier on the "watch" page.

Usually you should be able to tell if an identifier is actually needed. The strictly necessary ones are the first to be put in the link, so once you find an unnecessary one you can safely get rid of it and everything that comes after it.

Of course not all identifiers are evil, for example YouTube uses identifiers to specify that a video is part of a playlist:
https://youtu.be/dQw4w9WgXcQ?list=PLlaN88a7y2_plecYoJxvRFTLHVbIVAOoc
or for timestamps to make the video play starting from a specific point:
https://youtu.be/dQw4w9WgXcQ?t=43s

If there's more than one identifier and the first one is necessary (like with an old YouTube link) you'll be looking for an "&" instead of a question mark:
https://youtube.com/watch?v=dQw4w9WgXcQ&list=PLlaN88a7y2_plecYoJxvRFTLHVbIVAOoc&t=43s

But yeah, just keep in mind that if a link is longer than you'd expect it's probably because of a bunch of identifiers you can (and usually should) get rid of. Clean up your links!