The interesting thing that is not obvious to the general public (it certainly wasn't to me when I started) is that unless there is a medical condition that is extremel rare called cogential amusia, you can develop the skill. It's not easy, and some people definitely have more natural ability but it's learnable. You likely don't have amusia, as people that do have it have a hard time recognizing an instrumental section of a song. If you can recognize 'happy birthday' by someone humming the melody, you don't have it.

There are a bunch of ear training quizzes online and I believe also YouTube videos.

I don't think this is a general requirement for learning the guitar. It's just one aspect. For most people whether you can hear that something is a 3rd or a 5th shouldn't impact their ability to play songs and have fun on the guitar. A sense of rhythm is maybe more important. If you can whistle a tune or sing along anywhere close to a song you're probably ok.

I'd say the only way you're not cut out for an instrument is if you tell yourself you're not cut out for an instrument. Any other case will be a hyper rare exception.

This applies to most skills. How hard it's going to be or how far you'll go with sane effort may vary.