When interpreting Scripture, it is important not to interpret what God does not interpret. For instance, when Daniel explained to Nebuchadnezzar the frightening image of his dream, Daniel told him that he was the head of gold and that the chest and arms of silver was a kingdom that would come after his. Nebuchadnezzar did not ask, "Yes, but what is the neck?" or "What about the fact that there are two arms?" He just shut up and let the holy-spirit-infused wise man of Judah interpret what was relevant.

Likewise with metaphors. The rule of the metaphor is that there is one point of contact between two very disparate things—and only one. For instance, when the Lord says in John 15:5, "I am the grapevine," the right way to interpret this is to scan the context to find the point of contact. This is easy. The rest of the verse says, "He who is remaining in Me, and I in him, this one is bringing forth much fruit, for apart from Me you can do nothing." That's it: stick with Christ and you will bring forth fruit. Anyone looking for any other relationship between Christ and a grapevine, other than a grapevine bears fruit and those in Christ will bear fruit, is seeking to force his or her own interpretation upon the passage in order to forward a personal interpretation, or to artificially develop a "new teaching." For instance,  someone could say, "Christ said that He was a grapevine, NOT a goatskin of wine, therefore it is a sin to drink alcohol."

And so there are two ways to tell when someone is trying to force a personal interpretation or a "new teaching" on you: 1) they will seek to interpret what God does not interpret, and 2) they will push metaphors beyond their proper bounds. Whenever you see either of these things  happening: beware. The fruit produced by these practices is almost always error.