This will be small, but it's a neat little trick I was taught at the beginning of my music theory class.
Given that your song is in a Major key, it's VERY easy to figure out the key just by looking at the key signature. Without going through a huge long process by going through the Co5's (read blog below), you can figure it out easy.
For sharp key signatures, all you have to do is raise the LAST sharp a half-step. For example, let's use A Major. It has 3 sharps in it, F#, C# and G# in that order on the staff. The last sharp, as noted already, is G#. Well... what's G# up a half-step? A. Therefore, our key is A Major. For another example we'll use B Major. F#, C#, G#, D#, A# are all present in that order on the staff. Raise the A# a half-step and whaddya get? You got. B.
For flat key signatures, you look at the SECOND TO LAST flat in the key signature. Of course, this only works if you have at least two flats, so you'll just have to remember that "1 flat = F Major". So... we'll choose Ab Major for our example. Bb, Eb, Ab, and Db are there, in that order, on the staff. Look at the second to last flat in the key signature... it's Ab. Easy enough right? Good.
I'll have to come up with a way to tell the minor keys, but I've got another acronym for that... I use that right now, so I'll just go ahead and tell it:
The order of sharps and flats for minor keys are the same for major songs (F C G D A E B, and B E A D G C F, respectively). An acronym we came up with in 9th grade went as follows:
That IS the order of increasing sharps using the minor scale. E minor has one sharp, B minor has 2, F# minor has 3, and so on and so forth.
The only thing you have to remember with this one is that, after the F#, the tonic is sharped in EVERY key. See how it goes from B to F? It really goes from B to F# to C# to G# to D# to A#. Easy stuff right?
The only problem with this is, I don't have a way to remember it going BACKWARDS. I myself have to go "Destroyed, get, children, function.. aright, it's in F minor". So that'll be something I have to think of...
To go backwards, the only thing different (aside form what I said above) is that, once you hit the "B" key going backwards (adding flats) it's REALLY Bb Minor. It's Bb Minor because the flats it has in the key signature are as follows: Bb, Eb, Ab, Db, Gb. The next key is Eb Minor and then Ab minor. Make sense?
God I hope so.