Ok so this is how I know. Whenever I want to find what key a song is in this is what I do. You look at the last sharp, in this case it would be C#. What note comes after the last sharp? D comes after C. So the name of this key is D Major!!! I use this trick all the time since I play piano and violin and let me tell you it is such a life saver haha. Anyways hope this helps! Have a great day!!
