When reading a music sheet, how do I know what octave to play a note in for a guitar?

I'm trying to learn music theory for the guitar. I know how to read the notes and beats, but how do I tell what octave to play the note in? For example, the note E can be played with an open 6 string or on the 2nd fret of the D string. They're the same note, but they sound different because they're different octaves. So how do I tell which one to play from the music sheet? thanks!