If he does his job properly, all the notes sound 'in tune'. However, whereas each note can be tuned separately on the piano, a fretted instrument relies on an accurate fretboard. The tempered scale used on all instruments is a compromise. Put briefly, this means that all the individual notes are slightly out of tune, with the result that every scale in each key sounds more or less in tune. If your fretboard is inaccurately fretted you will not be able to play in tune all over the neck .
Worn frets can also cause notes to be out of tune.
A good professional luthier can check the intonation of each fret electronically. On a short scale instrument like the mandolin a compensated bridge is a necessity. This is because the standard fretting creates different results on each string, the differences being magnified by the short scale length. I'm not an engineer or a physicist, but what it amounts to is this. Stopping the string half way along its length raises the pitch an octave - in theory.