Perhaps if one is spending a great deal of time with a person it can be determined whether they are truly exhibiting evidence of salvation or not, but when encountering people only casually or even less, it may not be possible to know, at least not for an extended period of time.
I know plenty of folks I believe are saved that if one had looked at them at various points of their Christian life they might not have thought they were saved.
At the same time I know plenty of "good Christians" who appear to be evidencing the good fruit yet they have no biblical testimony of salvation.
Those we are closest to we should be able to tell where their heart is. We should be much closer to our fellow, local brothers/sisters in Christ but that's typically not the case, at least in America. Many of us only see fellow church members once or twice a week and very little, if any, actual contact with them. We can't know one another under such circumstance.
That is the problem with todays fellowships they are not personal enough for people to get to know each other.
large fellowships in Paul's day ended with people drawing followers after themselves.
1Cor 1:10 ¶ Now I beseech you, brethren, by the name of our Lord Jesus Christ, that ye all speak the same thing, and that there be no divisions among you; but that ye be perfectly joined together in the same mind and in the same judgment.
11 For it hath been declared unto me of you, my brethren, by them which are of the house of Chloe, that there are contentions among you.
12 Now this I say, that every one of you saith, I am of Paul; and I of Apollos; and I of Cephas; and I of Christ.
13 Is Christ divided? was Paul crucified for you? or were ye baptized in the name of Paul?
1Cor 3:3 For ye are yet carnal: for whereas there is among you envying, and strife, and divisions, are ye not carnal, and walk as men?
these people were comeing together having various teachings, take the Lords table in vain, and divided themselve into groups when they came together. Not much different than today in America.
We have big fellowships and only know a few because we have only one leader pretty much the senior pastor ( which is unbiblical) maybe a asst or two and youth pastor while the rest of the fellowship sit on their hands and leave the work to the few when all the body is to be fit together and working as a body and it is not.
American Fellowships are gathered around men who are hirelings, get a salary, 401k, insurance package, some get a home and a car to drive. and the more people they bring in the more they get. Ever seen how John Hagee has grown in waist size since he started. Now a days his ministry wont even give a dvd or book to a missionary who could use it for free. that is the American fellowship.