I suppose with separator, you mean centrifugal separator and with testing you mean testing the separator?
at your inlet you will have a composite liquid
at outlet you will have two separated fractions.
If the quality of a fraction is conserned,sampler should be mounted in that fraction outlet line .
If operational quality of separator is conserned,sampler should be mounted in composite liquid inlet line.
When both outlet fractions can be quantified, good working condition of separator can be judged upon.