A student asked me how you know what key of piece of music is in. The answer is: by the Key Signature.
The Key Signature is at the beginning of the piece of music on the staff where there will be between zero and five **sharps (#) or flats (b)**written next to the staff symbol.
# means raise the note by a half-step
b means lower the note by a half-step
The circle of fifths is a great tool for seeing and understanding key signatures. If you don’t have a circle of fifths, there’s a trick to telling the key by a the key signature.
The trick is:
For sharps: whatever note the last sharp is written on, the key is a half-step above that note. So just F is sharped = Key of G.
F and C are sharped = Key of D.
F,C, and G are sharped, Key of A.
For flats: if there is one flat it is the key of F, and for the rest of them the second to last flat symbol is on the note that the key signature represents.
So flats on B and E = Key of Bb.
Flats on B, E, and A = Key of Eb
Thoughts comments on key signatures and/or the circle of fifths?
In the end it’s no use to you and you have to go through… Without theory worse practice, I did it, with the help of my keyboard it went well. 
Greetings from Astrid 
