I have a hard time figuring out if a song with a certain key signature is major or minor. I know some sound minor and are and some sound major and are but some sound opposite of what they actually are(major sounds minor and minor sounds major). How can I tell if any song is major or minor?
In the middle of a song, it's difficult to tell if it's major or minor - at that point, it could be on any chord, major in a minor song, vice versa, and going somewhere else anyway ! Just stick to the last chord in a song to get the question answered. If recognising one chord in isolation is not easy, you need more exposure to blends. Get a friend to play and try to feel if it's a major or minor chord.Experience will help a great deal.
Try to understand the difference between C maj and C min. C maj and Amin.
Some songs will appear to be minor and end up major. Is 'Fly me to the Moon' maj/min ?
Can you find the root note?
Start with the root note and sing Doe a Deer. If "deer" sounds sharp or off then the piece is in minor.
In some degree it is possible to learn by training, but you need to have some chord-capable instrument and be able to get known chords out of it. Try to compare C versus Cm, E versus Em, A versus Am etc. Pick the middle of the sound range, not too high and not too low. After some practicing it should be possible for you to tell them apart. Minor chords always sound way more sad and romantic for some reason.
You do not need to identify all individual notes for that, you also do not need to separate between same type of chords (C from C# for instance) that is much more difficult task.
I'll elaborate on Tim's note from the above answers. First learn your relative major and minor keys. These are the major and minor keys that have the same key signature; the relative minor is always a minor third (3 half steps) below its relative major.
Now, look at the key signature. Which major or minor key are we talking about? Let's say the key signature has no sharps or flats. It is therefore either A minor or C major. Now, look for two things in the first few bars of the piece. Do they center around an a minor chord or a C major chord? Also, look for a G# occurring a lot. This is a raised 7th tone in a minor and is very common. If you see it, chances are good you're in A minor. If you don't, you might still be in A minor, if the music still centers around the A minor chord instead of the C major chord.
Keep in mind that a lot of music bounces back and forth between relative major and minor in the same piece. It's very easy to set up, and adds interest to the music. The best example of this that I can come up with off the top of my head is "O Little Town of Bethlehem" (the original American version of the tune, not the vapid Vaughan Williams "adaptation" of a British folk tune which the British inexplicably prefer to use). The piece is in a major key, but the part that goes "but in thy dark streets shineth the everlasting light" is in the relative minor.
This might not be the answer if you've never had any interval ear training, but it would be good to add it here.
What I usually try to look for is a V-I drop. In a lot of songs, it is used to determine the scale.
In some classical pieces, it used somewhere in the beginning of the song. For instance, if the song is in C harmonic minor, there would be a G - Cm drop. Whereas, if the song is in C major, there would be a G-C drop.That would help you understand the scale.
In C major and C harmonic minor scales, the V chord is the same. It's G-B-D. So, if I hear a G and then a C/Cm, I would know that the song is in C major or C harmonic minor scale respectively.
@AlexanderTroup, my teacher played a lot of V-I's on the piano, some where minor and some major. That was my ear training material. (There were more chords, not just V-I).
It is quite often seen as: IV-V-I (and in jazz, II-V-I)
But, in some more complex songs, this might not exactly point out the scale.