Learning scales really is useful. Apart from just knowing to play scales, you should also learn how to construct chords from scales (i.e. which notes of a scale to put together to form a chord). When you learn this, you'll figure out exactly which chords go into which scale and when you'll see a chord progression you'll immediately know which key the song is.
Here's a quick example..
Let's say you wanna tab out a song and you know that the chords are C Am F and G.
Now take a look at which chords consist of which notes:
Looking at the notes of these chords you can see that only the notes of the C Major scale were used to make them (C D E F G A B ).
Or even better yet. The easiest way (and usually correct) way to figure out what key a song is in is this: listen to the very last chord of the song, the one the song resolves to. When guitar chords are displayed this usually isn't there but, for example if you would play the song Those Magic Changes and finish on the G Major chord you will leave the song hanging in the air, waiting for it to resolve on the C (three times in this particular song to do the cha-cha-cha thing
) which is the key of the song.
One other way to do it is to open a picture of the circle of fifths. I have it on a poster on my wall so now it's engraved in my memory, and I think it should be in every musician's head. It's really really useful. Here it is:
Let's say the song is in C major. All the basic triad chords that can be formed from C major are directly beside C major in the circle of fifths:
C, Dm, Em, F, G, Am (there's another chord, it's B diminished)
Some of thse chords are major and some are minor because some have a major 3rd and some a minor third.
But you'd do well to remember this formula:
I ii iii IV V vi VIIdim
So if you know the notes of any major scale you can construct the chords just by using this formula (uppercase roman numerals mean it's a major chord, lower case mean it's a minor).
So here's an example for the D major scale:
Here's the scale:
D E F# G A B C#
By applying the formula, I can get the chords:
D Em F#m G A Bmin C#dim
So really the only thing you need to know is how to learn the notes of any scale. I have memorized which scales have which sharps and flats and can identify them by that. I'm sure there's other ways as well
Hope this helps a bit