This illustration shows the alpha characters for the key as well as the G clef key signature accidentals. Being able to determine which key as song is played requires the knowledge of accidentals and their use. You must learn how to and where notes are placed in the staff to help you identify the key signature.
I am going to start posting more music theory on this site in an effort to help people with the basics of music. In essence, it all starts here. If you want to get an idea of how the circle has been utilized to genius level - check out this John Coltrane song with a genius graphic depiction of the project scales and modes of the song.
I hear a lot of people who can play one part of a song well but cannot play entire pieces or the piece they play in the music store is the extent of the musical repertoire. Don't be that person and strive to expand your knowledge because knowledge IS cool and will never go out of style. Sure, many greats lack theoretical musical experience. I am certain the average person could name hundreds of musicians who made a living without knowledge of music theory but the fact is that there are over a billion people who don't know it and will never be great. Learn to like it and consume it - Then forget it and be yourself. However, there is no substitute for practice!