how do you now the root notes of like g major of c major of something????
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The root is the first note. The root of G major is G, C major is C etc.

you can never know the root simply by looking at the scale, you can only guess that the first is the root....

To know the root, you must know the tonal centre of the musical part which may actually be produced by a seperate rythmn set or guitar.

You can learn scale shapes which can easily be applied to different 'keys', you just need to match up the scale shape with the key, for example you would play this shape over a G major key if you wanted to-

stop making new threads? reported.
and instead of asking such basic questions, check out the theory FAQ.