Sight reading scores for instruments with controls that are not diatonically organized like the score is, is cumbersome. That is one reason that certain wind instruments are "transposing", writting in a different pitch than played. The problem is exacerbated for polyphonic instruments.
That is the reason that there are instrument specific notations like "tablature" (which were used even in Renaissance and Baroque periods for lute and some other string instruments).
It's also probably the reason that countries with more of an "aural" rather than a "written" accordion music tradition (namely people playing by ear rather than by score) tend to employ chromatic button accordions, while things like "accordion orchestras" use the bulkier and less versatile piano accordion.
Of course, you can ask even a hobbyist guitar player or a hobbyist chromatic button accordion player "can we do this a minor third higher?" and he'll oblige (the former after reaching for a capodaster but so what).
With the more score-friendly instruments, you'll be separating the wheat from the chaff with that question.
So the point is: playing guitar from a score is a non-trivial effort: it is a polyphonic instrument not matched to the diatonically organized score. You acquire it like reading: the chord patterns have to make independent sense on the paper before you form the respective pattern with your fingers. Before the words/vocabulary becomes familiar units that are different entities on paper, in your fingers, and in sound, you'll be stuttering.
Tablature is easier to get accustomed to but less versatile. You can't, say, ask some keyboard player to play two bars for you so that you can listen and get the idea.
At any rate: guitar scores often have fingerings in them (as others mentioned) and if you learn to read them, you at least have the information required to play (even if you can't do it right off the sheet). When fingerings are in the lowest-numbered frets/strings possible, they are often left out.
When there are no fingering/positioning instructions at all, figuring the best out is left as an exercise to the player. Usually you'll play in the lowest possible position since that puts the empty strings in natural places in the scales and tends to work best for assembling chords, but of course not everything works out in that manner.