Chasing goats or leading the sheep?

I once heard an evanglist say quit chasing the goats and tend to the sheep. I have seen where pastors spend all their time with certain families who proved to be unfaithful concerning the things of the Lord, while those who were truly seeking the Lord, were ignored and brow beaten for wanting the sincere milk of the word.

When we are dealing with people (even those that claim to be Christian), how can we discern their true heart and know when to leave well enough alone.