My last post raised a question that I was thinking about last night. ds asked a variant of it in the comments, and it turns out that Calvin addressed it a few weeks later in his preaching (can you guess one of the books I'm reading at the moment?).
Roughly put, the question is this: Given that most false teachers mean well, how can you tell the difference between a misguided minister (like Apollos was in my Acts 18 quote, and like I am sometimes too), a false teacher (boo, hiss), and a good minister?
The good minister / misguided minister distinction is the hardest, because people don't tend to be infallible, which means that many ministers may well be misguided in some respect. But the misguided / false distinction is easier to spot.
If we desire to be Christians, let us honour the Son of God by continuing to listen to his Word and obey it, even when we do not like what it says and when our natures find it unpalatable. Thus, when anyone comes to a sermon, above all else he should be ready to be rebuked when necessary, and he should realise that if he is not comforted by it, then it is to his profit... Let us all be willing to have our wounds scratched, as it were, and to be condemned, and to hear the opposite of what we would like to hear. This is how we should prepare ourselves to be good scholars under the Son of God, and to attribute to him his rightful mastry over us. We must work all the harder at this if we see that our natures are pushing us to do the opposite, for we are often blinded by self-love.
John Calvin, Sermon on Galatians 1:8-10
In other words, I think the observable difference is that a false teacher will carry on teaching stuff, even when confronted with the fact that Scripture teaches otherwise. A true minister, when misguided (as Apollos was) will be open to correction from Scripture, when done lovingly and appropriately.
The key difference then is whether they seek to follow what the Bible says, in which case they're ok, but they might be horribly misguided and / or not gifted for the job they are meant to be doing, or whether they seek to teach their own opinions, in which case they're a false teacher, even if their own opinions are fairly close to what I think the Bible says.