At this point you should know
how to tell what chords are in a Major key/scale.
But, you'll find that "songs" don't necessarily conform directly to this thinking...IOW, they don't always look like they are completely part of a "Key".
I'll deal with a lot of the background of this stuff in my tutorial but this is a little Primer on "what key do I play in".
For a major key, building a chord off of each note, starting with the first note in the scale, you will end up with this pattern of chords/triads:
I - major
ii - minor
iii - minor
IV - major
V - major
vi - minor
vii° - diminished
So say you were in the key of C, just plug in the C major scale:
I - C major
ii - D minor
iii - E minor
IV - F major
V - G major
vi - A minor
vii° - B diminished
After you learn this you need to progress to the 7 and extended chords to really be able to pick keys out.
In the Key of C:
Imaj7 - Cmaj7
IIm7 - Dm7
IIIm7 - Em7
IVmaj7 - Fmaj7
V7 - G7
VIIm7b5 - Bm7b5
The easiest way to determine a key is logic, process of elimination, etc...
Look at the IV and the V: this is THE ONLY time two major chords are a whole-step apart in a key. So, anytime you see two major chords a whole-step apart...it's a good indication that they are the IV and V of a key.
Look at the ii and iii chords: This is the ONLY time two minor chords are found a whole-step apart in a key. To dig in deeper try flatting the 5th of the high of the two chords...if it works...Then these two chords could be the vi and vii chord of a progression. But if that flat 5th doesn't sound right, then they are the ii and iii of a key. Got it?
Look at the V7: The dominant 7 happens ONLY once in a key. So, anytime you play a dominant 7 chord...it can be considered the V chord of a key. How about what we call a I7-IV7-V7 progression??? You can look at each of those chords as being the V7 chord of a different key. So, those three chords shouldn't be thought of as "being in one key", but three different keys. Try it...you'll like it, trust me.
Look at the maj7 chords: anytime you run into a maj7 chord, it can only be the I or the IV of a key.
These are very simple but very useful ways to determine a key. I still use them all the time.
Now, when you throw substitutions, key changes, etc...on top of this you can see where you have to be able to think on your toes. I can show you some complex examples of this but they can usually all be figured out by using those guidelines above.
Hope this helps a little bit.
Genn always says, "once you know the chords, the scales are a given"