songs key

could anyone tell every method to recognise the key of a song? For example in Wild Horses I have G, Am, Bm, C, D, F. So, I think the key is G cause it's the first chord played and, except for F, the major chords are GCD which make the I IV V progression and the rest of the chords are played minor. Well, what I wrote is pretty confusing , but as I said before could anyone tell me every way to recognise the key of a song? I would be great if it is explained with a clear example.
thanks!

Usually a song's key will be found with the chord that seems to 'attract' every other note. That chord is usually the root (or first note) of the scale used in the tune.

If you play a chord scale, you get (in G major)
G Am Bm C D Em F#mb5
That's pretty close to what you have, right? The last chord is pretty weird, it's not really used in pop or rock. Your progression is in G. The F can be explained by many different chord substitution, too long to explain right now.

Anyway, in Major, you always have that kind of progression
I IIm IIIm IV V VIm VIImb5

And in Minor, you start with the 6th note

Im IImb5 III IVm Vm VI VII

That's the basis for modes too! The first is a Ionian progression, and the second an Aeolian one.

thanks man! are you a teacher or something? you know lot of theory and you play excellent

I teach music to kids from 3 to 11 years old, so I guess it has given me a way to explain things. I'm also a guitar teacher for grown ups, and I've seen that they sometimes need that kind of simplifying to get it.

But I'd love doing sessions for a living, at least for a while. I still do some, but not enough...