best way to find modes and scales on guitar?

hi. im playing for a while and i have started learning theory for composing. as you know most of the composers are pianists or better say their first instrument is piano because place of notes are obvious and you can find scales easy on it and all my theory teachers teach theory on it. but as a guitarist specially electric guitarist that my first beloved ganre is rock and heavy metal i like to make melodies on guitar and i dont think most of the rock bands make song and melodies on piano or keyboard and move it guitar but there is a problem. how should i find scales and modes on guitar. in many guitar books they introduce some patterns like they name it caged pattern but i think its not sufficient and in professionl mode no one do that because for some techniques like sliding you cant move your hand at all. for example i want to know how joe satriani made "forgotten" as you see his hand moves to all parts of guitar. does he know notes of all sacles in all part of the guitar or just main scales like do major and la minor and after making all of the song he just transposes to other scales. as you know more than 90 precent of scores we see is on do major or la minor. i want to know what most of the great bands do. thank you for helping