I think this is the trickiest part of the whole deal. Looking at people's fruits, which by all accounts, seem to be genuine love, joy, peace, patience, and kindness, with no hypocrisy or envy, and yet, they believe erroneous doctrines. How can they possibly exhibit all these fruits while walking in blindness and error? I'm still trying to figure it all out myself.
I think someone above brought up a good point about people not really knowing themselves, what's in their heart. I know that's how I was. What I thought was righteousness, I now see as nothing more than my own works of the flesh. Sure, I could say that it was all the work of God, and I actually THOUGHT I believed it, but did I really believe it? No. Now I have a better understanding of God's sovereignty and my complete ineptitude. Now that I see this, I can actually better analyze others and kind of see whether or not they may be deceiving themselves as I was. But I'm not yet in any position to go about pulling specs out of people's eyes. Yeah, I may have had my plank removed, but I still have too many weaknesses and honestly, too much cowardliness to go around showing others' their shortcomings. People won't like that and I'll lose friends and create icky situations. I know that all of that comes with the tribulation of entering the kingdom, but I confess that I'm not ready for all that yet, so I just have to wait for God to take care of things.
I believe this all relates to what Paul wrote somewhere (forgive me for being lazy and not looking up the scripture); Paul wrote something to the effect of "In a house there are many items. Some are precious stones and some are wood and stubble. If a man cleanses himself of the latter, he will be equipped to do the works that God requires." I'm sorry for the weak paraphrase, but hopefully you get the point. I believe the first step is having our plank removed, and then comes the part of overcoming weaknesses of the flesh. I can't very well go about pointing out others' weaknesses when I've still got a ton myself, otherwise I'd be nothing more than a hypocrite. So I'd rather just confess to being a cowardly sinner and wait for God to do his thing; I'd rather do that than go about being a hypocrite.
Sorry, I began rambling there.
Getting back on topic, sometimes I think it takes careful inspection and really what it all boils down to, is God-given discernment. It reminds me of when Jesus condemned the Pharisees for having the outside of the dish clean, but the inside was full of filth. Many times, people will exhibit all these fruits, but if or when you begin to challenge or question their doctrines, you'll see the fangs start to come out a little bit. And I'm not talking about pridefully or loudly rebuking them, I'm talking about just sticking to what you believe or have been shown. Many times they won't take kindly to it, no matter how humbly or peacably you talk to them.
Also remember, many people of other religions and even atheists exhibit all kinds of admirable traits and qualities, such as selflessness, charity, humility, and so forth. Even dirty minded, godless soldiers lay down their very lives for friends and country. That's not to belittle them or take shots at them. But it just goes to show how tricky it is to discern true acts of Godliness done in Truth, from great human acts that are not of God. You'd think that dying for someone is verification of true humility and selflessness, and even Christ said that's the greatest love that someone could have. But we know (from the Iraqi prisoner scandal a few years ago, amongst other things) that many or most of the soldiers who are dying for others are quite godless. Again, this is not to condemn them, because I certainly appreciate their willingness to die for others, but in reality, it is nothing more than an act of the flesh, the beast. It's hard to grasp and accept, but I believe it's true.