Ref No. 2 (quick to disconnect and check DI)
By unclipping a female NITO fitting from the male fitting, then turning the water flow back on again to fill a cup to test the TDS, switching off the water and connecting the fitting pair back up again.
Rather than unscrewing or loosening off the jubilee clamp on the hose attached to the barb fitting on a normal plastic fitting type, removing the hose to then check TDS, then connecting it all back up again and re-tightening the jubilee clamp or fitting a new 'O' clip.