There are no correct answers to this question because it will depend on your hearing ability and how advanced you are in knowledge. This means that we can’t give you a magic solution but we can explain to you which are the process to achieve what you want, how to know what key a song is in will help you if you want to modify it, or if you have to take an exam. Anyway, there are several options and we will try to show them to you in the simplest way possible so that you can start practicing.
Listening is the main key.
If you don’t listen to a song you won’t know anything about it, but if you repeat it over and over again you’ll be able to identify details that you might have missed. These details are what define the main key of a song. It’s a constant sound that can be identified once you know what you’re looking for. You can’t find something you don’t know, so you have to educate your ear to be able to notice the subtleties in a song.
Ask other musicians.
When you think you have found the key of the song, check with others to be sure. If these people don’t agree with you, they can always propose a reunion to share information. There is not more than one correct answer but if there is more than one opinion and one method to find the key, you should take advantage of the knowledge of others and share your own, and the easiest way to find it so that you can finally get some rest. Some people are choked by the idea of not being able to achieve things at first attempt but this cannot be avoided in music, everything is about to try.
Check on the Internet how to know what key a song is in.
If after a lot of trying you feels that you can’t get a correct answer, the best thing is to look for the answers on the Internet, identify where your failures were and start again. There is no way to skip over failed attempts so it’s best to learn. When one song saturates you it’s best to move on to another, but first, you have to look for the answers to identify what you’re failing at. It’s a simple but infallible recipe.
Inside the harmonic analysis, you will find answers.
This is a slightly more complicated way but it may be perfect for you. If you are already familiar with notes and chords, you can try to modify some details of the song and see what changes, until you find the right answer. You must cultivate your patience and start practicing right now to become a talented person you want to be. Grabbing your guitar and starting to practice eliminates stress and also allows your ears to get in tune with your hands, but none of this will happen if you don’t have classes supervised by qualified teachers.