Thought on the Wheat and the Tares

I am preparing to preach on the wheat and the tares (Matthew 13:24-43) in two weeks. As I studied something stood out to me. I have often been told that one point of the parable is that you cannot tell the wheat from the tares. We must be careful. There will always be those among us who look like Christians, but are not. Christ will sort them all out at the judgment day. But until then we must be cautious in assuming too much.  However, there are some problems with that interpretation.

First, Jesus plainly says in Matthew7:15-20 that we can tell who is wicked and righteous by what fruit they bear. He says something very similar in 13:18-23. He is talking about false prophets, but the principle applies across the board. (See Luke 6:43-45). The tares may be self deceived (Matthew 7:21-23), but God's people, and his ministers, should not be. They should use biblical criteria to evaluate whether someone is a wheat or a tare.  

Second, and closer to the story, in 13:27 the servants plainly know that the tares are in the field.  The servants can see the difference between the wheat and the tares even though the harvest has not yet come.

I am not yet sure what to do with this data as it relates to preaching the sermon. But I do think it undermines the false notion that in this life we cannot tell the wheat from the tares.