We've both been around this forum long enough I might have known you were an HD person.
Even if that isn't true, I've been here long enough to know better than to assume MD.
A quick way to tell if a tune is in a major key is to look at the last note. If that is the same as the key note the tune is usually
in that major key. This isn't foolproof, but is a good rule of thumb.
How do you tell the major key associated with the key signature? Use the following rules:
1) Sharps are always added to the key signature in order. The order is FCGDAEB. In the key signature, the last one added will be furthest to the right.
2) Similarly, flats are added in order (and will show in the key signature in that order). This order is the reverse of the order of the sharps, i.e. BEADGCF.
3) If you have sharps, the key note is the next note above the last sharp added. For instance, One sharp is key of G (the one sharp is F). Two sharps i D (the sharps are F and then C).
4) If you have flats, one flat is key of F (trust me). After that, the rule is the key is the next to last flat added. For instance the key of B-flat is 2 flats (B then E). The key of D-flat is 5 flats (B, E, A, D, and G).
5) If there are no sharps or flats, it is the key of C.