realisation of tuning in performance

Composers who build purpose-built instruments particularly fretted stringed or tuned percussion intruments seem to have a fairly straight forward way to guarantee the integrity of a particular tuning system. However when classically trained singers or players of fretless string instruments perform
music that is allegedly in some form of just intonation, I wonder how often a performer's realisation deviates from the theoretically ideal tuning prescribed by a composer. In the case of music where intervals are heard vertically (or simultaneously) rather than horizontally (or sequentially), what
would be the most objective way of knowing when a performer has reached the tuning prescribed by the composer ? How would one go about analysing a performance to determine if the tuning was more like one tuning than another ? In other words while we can know what they are supposed to sing or play, how
do we demonstrate when they are actually doing it ? This seems like an important question to me. I hope it's not a silly one.

Greg S