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Triads in Music and the V7 Chord
All triads in music (except diminished and augmented) are consonant. All 7th chords are dissonant. The consonant V chord (five chord) triad can be converted into a dissonant V7 chord by adding the 7th note of the scale to the chord. So, getting back to the problem of ambiguity inherent in the progression ...
C – G – C – pause – G – C – G – pause – G – C – G
... all you need to do is turn the V chord into a dissonant V7 chord.
In the above example, if the G major chord were converted into a dissonant chord, your brain would know for sure that the key could not possibly be G major. That’s because the tonic chord is always a consonant triad.
So, to convert that consonant V chord to a dissonant chord, the simplest thing to do is to add another note, converting it into a dissonant V7 chord (“five-seventh,” in Nashville Number parlance).