More On What Key Are You Playing In

Over 95 % of the time, a song will start and end on the same chord and that is usually the key.

There are exceptions but they are rare. For songs that fade out, check out the first chord in the song and the last chord at the end of the chorus. They will usually match up, and give you the key.

Where the first chord and last chord are different, you can usually rely on the first chord as the key.

This is hardly academic but it works way more often than it doesn't.