Tenor and guitar parts are conventionally written one octave higher than sounded. In correct notation, the 8 below the clef indicates that the sound is one octave lower than the notation. This is important for playing the tenor part at actually sung pitch. In two-staff notation for four-part harmony, tenor and bass are notated together in a single bass clef and the tenor is then written at pitch.
The "correct" presence of the "8" is more dependable the more recent the music is. Modern editions are pretty reliable in that respect.
Particularly in Renaissance and other old music typically performed by male singers only, the tenor part may actually be written in proper treble clef occasionally. This will usually be discernible by
a) other movements having the 8 subscript in the tenor part
b) the range occupying significant stretches of the lower part of the staff rather than the upper
In contrast to the occasionally murky tenor situation, guitar scores are always played one octave lower than treble clef, regardless of whether the writer thought of including the 8 or not.