Some do not discern. Because they think of Matthew 10:42 where Jesus commend those who give a cup of water. But there is a portion omitted: "...because he is my disciple..." or "...you belong to Christ..."
Therefore, one test which cannot be always executed, is to see how they take a good rebuke. If they will not accept the rebuke, it is clear where their motive lies.
Other tests could be formulated based on the above: reaction to preaching, presence/absence of disagreement, etc.