Lukitoh:

The final (last) chord of the song (typically a I chord in 99% of all songs) is almost always the 'key' of the tune as well. To determine the key of a song (in my head), I'll sing/play the last few measures of the song and figure out the last chord. The last melody note is most often (though not always) the root, 3rd, or 5th of the chord. The chord preceding the last chord is usually (in pop music) a V7 or IV chord.

I recommend listening to the final cadence of the song to hear how the V7 or IV resolves to the I (tonic) chord (key of the song).

Hope that helps.

Scott