Theory; finding what key a song is in (need help). (quote, guitar)

[Skinnyboy23]

Hey there, this is my first post, so don't go too hard on me.
I just started learning music theory on my own; I play guitar, for the most part. I taught myself modes... learned intervals from a guitar teacher back in the day... and know the major scale, minor scale(who's even counting that), and pentatonic scales (same with that). So, as you can see, I don't know much music theory, haha

That's why I've searched for help!

I've always asked my friends, that play music as well, how to tell what key a song is in... none of them ever give me a theoretical explanation. Some said whatever chord is played the most... YEAH RIGHT!... is what I thought to that.

So, with that said... how does one know what key a song is in?

This is my hypothesis, with little knowledge of music theory I have...

Whatever major scale the song is in; the root note of that scale is the key of the song.

Is this right?

Anyways, thanks to anyone who can be of some help towards this!

Easy days