The odds of getting a bad apple would seem to dictate that if they exist, they are tiny (dozens at most). And it would be virtually impossible to tell.
The tares do often serve some kind of constructive function in the churches; God doesn't just have them there to frustrate the wheat. The proof is that he will not uproot them now "lest he hurt" the wheat.
It appears then, that the responsibility for being holy, rests with the individual. You never know for sure, if the person sharing the pew with you, is for real or not. Even the priest. Even the bishop. Even the Pope. It looks like you are on your own. Then why have a church? Would it not be better just to believe on your own?