Here are some of the best words I have read (or heard) on how a Christian should judge the spiritual condition of another professing Christian. They are taken from the Religious Affections, pages 110-111.
The true saints have not such a spirit of discerning that they can certainly determine who are godly and who are not. For though they know experimentally what true religion is in the internal exercises of it, yet these are what they can neither feel nor see in the heart of another…They commonly are poor judges and dangerous counsellors in soul cases, who are quick and peremptory in determining persons’ states, vaunting themselves in their extraordinary faculty of discerning and distinguishing in these great affairs, as though all was open and clear to them. They betray one of these three things: either that they have had but little experience; or are persons of weak judgment; or that they have a great degree of pride and self-confidence, and so of ignorance of themselves. Wise and experienced men will proceed with great caution in such an affair.
When there are many probable appearances of piety in others, it is the duty of the saints to receive them cordially into their charity, and to love them and rejoice in them, as their brethen in Jesus Christ.
From the Study 