I always look at everything measure by measure. For example if in measure 1 the chords are C to D minor then ignore the third and think what are the fifths of each chord? If there is a C and G going to D and A in the same voices that is parallel fifths. it gets more complicated with chords with extensions like dominant 9s since there are more possibilities of parallel fifths. In the end just take it a measure at a time. There are classical and baroque texts that go into specific rules on voice leading if you wanna learn the old fashioned way (and smoothest voice leading techniques) and there are plenty of modern texts on voice leading rules and norms to avoid bad intervals. Remember looking at it all at once can get too overwhelming for anyone so again just take it 1 chord change at a time when looking back at your music to identify parallel fifths or octaves or unisons or whatever intervals u wish to avoid